Chapter 29

[235]Sheet 101 S. E. of the Geological Survey of England and Wales and Explanation illustrating the same; and papers by him inQuart. Journ. Geol. Soc.vols. xxxi. xxxii. (1875-76). See also Messrs. Aveline and Hughes,Mem. Geol. Survey, Sheet 98 N.E. (Kendal, Sedbergh, etc.).

In estimating the area over which the volcanic rocks of the Lake District are spread, geologists are apt to consider only the tract which lies to the south of Keswick and stretches southward to a line drawn from the Duddon Sands to Shap. But it can easily be shown that this area falls far short of the extent of that wherein the rocks can still be traced, and yet further short of that over which the lavas and ashes originally spread. For, in the first place, the volcanic group can be followed round the eastern end of the mountain-group which culminates in Skiddaw, and along the northern base of these heights to Cockermouth, though only a narrow fringe of it emerges from underneath the Carboniferous series. It is thus manifest that the volcanic rocks once stretched completely across Skiddaw and its neighbours, and that they extend northwards below the Whitehaven Coal-field. But, in the next place, far beyond these limits, volcanic rocks, which there can be little doubt were originally continuous with those of the Lakes, emerge from beneath the base of the Cross Fell escarpment,[236]and still further to the east a prolongation of the same group rises for a brief space to the surface from under the great limestone sheets of Upper Teesdale. Between the north-western and south-eastern limits within which the rocks can now be seen there intervenes a distance of some 11 miles, while the extreme length of the tract from south-west to north-east is about 50 miles. Even if we take these figures as marking the approximate boundaries of the region covered by the volcanic ejections, it cannot be less than 550 square miles. But this is probably much less than the original area.

[236]For an account of the Cross Fell inlier of Silurian rocks see the paper by Professor Nicholson and Mr. Marr, with the petrographical appendix by Mr. Harker.Quart. Journ. Geol. Soc.vol. xlvii. (1891), pp. 500, 512.

The thickness of the accumulated volcanic materials is proportionate to the large tract of country over which they have been spread. From various causes, it is difficult to arrive satisfactorily at any precise statement on this question. In a volcanic series bedding is apt to be obscure where, as in the present case, there are no interstratified bands of ordinary sedimentary strata to mark it off. It tends, moreover, to vary considerably and rapidly within short distances, not only from subsequent unequal movements of subsidence or elevation, but from the very conditions of original accumulation. Mr. Ward considered that the maximum thickness of the volcanic group of the Lake District might be taken to range from 12,000 to 15,000 feet.[237]Professors Harkness and Nicholson, on the other hand, gave the average thickness as not more than 5000 feet.[238]My own impression is that the truth is to be found somewhere between these two estimates, and that the maximum thickness probably does not exceed 8000 or 9000 feet. In any case there cannot, I think, be much doubt that we have here thethickest accumulation of volcanic material, belonging to a single geological period, anywhere known to exist in Britain.

[237]Ward,op. cit.p. 46.[238]Brit. Assoc. (1870) Sectional Reports, p. 74.

[237]Ward,op. cit.p. 46.

[238]Brit. Assoc. (1870) Sectional Reports, p. 74.

The geological age of this remarkable volcanic episode is fortunately fixed by definite palæontological horizons both below and above. The base of the volcanic group rests upon and is interstratified with the upper part of the Skiddaw Slate,[239]which from the evidence of its fossils is paralleled with the Arenig rocks of Wales. The highest members of the group are interstratified with the Coniston Limestone, which, from its abundant fauna, can without hesitation be placed on the same platform as the Bala Limestone of Wales, and is immediately followed by the Upper Silurian series. Thus the volcanic history comprises the geological interval that elapsed between the later part of the Arenig period and the close of the Bala period. It begins probably not so far back as that of the Arenig group of Merionethshire, and its termination was perhaps coincident with the dying out of the Snowdonian volcanoes. But it contains no record of a great break or interval of quiescence like that which separated the Arenig from the Bala eruptions in Wales.

[239]Mr. Dakyns has expressed his belief that the volcanic group lies unconformably on the Skiddaw Slate (Geol. Mag.1869, pp. 56, 116), and Professor Nicholson has formed the same opinion (op. cit.pp. 105, 167;Proc. Geol. Assoc.vol. iii. p. 106). Mr. Goodchild, however, has shown that in the Cross Fell inlier the oldest tuffs are interstratified with the Skiddaw Slates (Proc. Geol. Assoc.vol. xi. (1889), p. 261). Mr. Ward in mapping the district inserted a complex series of faults along the junction-line between the volcanic series and the Skiddaw Slates. When I went over the ground with him some years before his death I discussed this boundary-line with him and could not adopt his view that it was so dislocated. More recent re-examination has confirmed me in my dissent. A large number of the faults inserted on the Geological Survey map to separate the Skiddaw Slates from the Borrowdale volcanic series cannot be proved, and probably do not exist. Others may be of the nature of "thrust-planes." But see Mr. Ward's explanation of his views,op. cit.p. 48.

The materials that form this enormous volcanic pile consist entirely of lavas and ashes. No intercalations of ordinary sedimentary material have been met with in it, save at the bottom and at the top. The lower lavas, well seen among the hills to the south of Keswick, were shown by Mr. Ward to be intermediate between felsites and dolerites in regard to their silica percentage, and he proposed for them the name of felsi-dolerites. They are comprised in the group of the andesites or "porphyrites." From the analyses published by Mr. Ward, the amount of silica appears to range up to about 60 per cent.[240]They are usually close-grained, dull dark-grey to black rocks, breaking, where fresh, with a splintery or conchoidal fracture, showing a few minute striated felspars, apt to weather with a pale-brown or yellowish-grey crust, and sometimes strongly vesicular or amygdaloidal. They present many external resemblances to some of the "porphyrites" or altered andesites of the Lower Old Red Sandstone of Scotland. A microscopic examination of specimens collected by Dr. Hatch and myself from the hills to the south of Keswick showed the rocks to be true andesites, composed of a multitude of slender laths (sometimes large porphyritic crystals) offelspar with a brownish glassy groundmass, and with some chloritic material probably representing augite, but with no trace of quartz.[241]

[240]Quart. Journ. Geol. Soc.vol. xxxi. (1875) p. 408, vol. xxxii. (1876) p. 24. Geology of Northern Part of Lake District (Mem. Geol. Survey), p. 22. In a subsequent paper the more basic lavas of Eycott Hill are compared with dolerites (Monthly Microscopical Journ.1877, p. 246).[241]These rocks were mapped as tuffs by Mr. Ward. Their microscopic characters have been described by Messrs. Harker and Marr,Quart. Journ. Geol. Soc.xlvii. (1891), p. 292; by Mr. Harker,op. cit.p. 517; and by Mr. W. M. Hutchings,Geol. Mag.1891, p. 537; 1892, pp. 227, 540.

[241]These rocks were mapped as tuffs by Mr. Ward. Their microscopic characters have been described by Messrs. Harker and Marr,Quart. Journ. Geol. Soc.xlvii. (1891), p. 292; by Mr. Harker,op. cit.p. 517; and by Mr. W. M. Hutchings,Geol. Mag.1891, p. 537; 1892, pp. 227, 540.

Another type of andesite has been found by Mr. Hutchings to occur abundantly at Harter Fell, Mardale, between the Nan Bield Pass and High Street, and in the cliffs on the right side of the Kentmere Valley. It consists of rocks mostly of a grey-green or grey-blue colour with resinous lustre and extremely splintery fracture. They are augite-andesites of a much more vitreous nature than the dominant type of lavas of the Lake District. Their groundmass under the microscope is seen to have originally varied from a wholly glassy base to an intimate mixture of glass and exceedingly minute felspar-microlites. This groundmass is permeated with chlorite in minute flakelets, and encloses numerous porphyritic sharply-defined felspar-crystals, together with chlorite-pseudomorphs after augite.[242]Gradations from these rocks to the ordinary more coarse-grained andesites may be observed.

[242]Mr. Hutchings,Geol. Mag.1891, p. 539. This observer describes a quartz-andesite or dacite from near Dunmail Raise.

Some of the andesites appear to have a trachytic facies, where the felspars of the groundmass consist largely of untwinned laths and appear to be mainly orthoclase.[243]

[243]Op. cit.p. 543.

Among the lavas of the Lake District there occur many which are decidedly more basic than the andesites, and which should rather be classed among the dolerites and basalts, though they do not appear to contain olivine. These rocks occur at Eycott Hill, above Easedale Tarn, Scarf Gap Pass, Dale Head, High Scawdell, Seatoller Fell and other places. Analyses of those from Eycott Hill were published by Mr. Ward, and their silica percentage was shown to range from 51 to 53·3.[244]The microscopic characters of the group have been more recently determined by Mr. Hutchings[245]and Messrs. Harker and Marr.[246]

[244]Monthly Microscopical Journal, 1877, p. 246.[245]Geol. Mag.1891, p. 538.[246]Quart. Journ. Geol. Soc.vol. xlix. (1893), p. 389. Mr. Harker,op. cit.vol. xlvii. (1891).

[244]Monthly Microscopical Journal, 1877, p. 246.

[245]Geol. Mag.1891, p. 538.

[246]Quart. Journ. Geol. Soc.vol. xlix. (1893), p. 389. Mr. Harker,op. cit.vol. xlvii. (1891).

The andesitic and more basic lavas are particularly developed in the lower and central part of the volcanic group. They rise into ranges of craggy hills above the Skiddaw Slates, and form, with their accompanying tuffs, the most rugged and lofty ground in the Lake District. They extend even to the southern margin of the volcanic area at one locality to the south-west of Coniston, where they may be seen with their characteristic vesicular structure forming a succession of distinct flows or beds, striking at the Coniston Limestone which lies upon them with a decided, though probably very local, unconformability.[247]One of the flows from this locality was found by Dr. Hatch, under the microscope, to belong to the more basic series. Itapproaches a basalt, containing porphyritic crystals of fresh augite instead of the usual felspars, and showing a groundmass of felspar microlites with some granules of augite and dispersed magnetite. This local increase of basic composition is interesting as occurring towards the top of the volcanic group. A porphyritic and somewhat vesicular andesite, with large crystals of striated felspar in a dark almost isotropic groundmass, occurs under the Coniston Limestone near Stockdale.

[247]This unconformability has been described and discussed by various observers. The general impression has been, I think, that the break is only of local importance. Mr. Aveline, however, believed it to be much more serious, and he regarded the volcanic rocks which were ejected during the deposition of the Coniston Limestone series as much later in date than those of the Borrowdale group. SeeMem. Geol. Survey, Explanation to Sheet 98 N.E. 2nd edit. p. 8 (1888).

Mr. Ward was much impressed with the widespread metamorphism which he believed all the volcanic rocks of this region had undergone, and as a consequence of which arose the difficulty he found in discriminating between close-grained lavas and fine tuffs. There is, of course, a general induration of the rocks, while cleavage has widely, and sometimes very seriously, affected them. There is also local metamorphism round such bosses as the Shap granite, but the evidence of any general and serious metamorphism of the whole area does not seem to me to be convincing.[248]

[248]The metamorphism of all the rocks, aqueous and igneous, around the Shap granite has been well worked out by Messrs. Harker and Marr,Quart. Journ. Geol. Soc.vol. xlvii. (1891) p. 266, xlix. (1893) p. 359.

With regard to the original structure and subsequent alteration of some of the andesitic lavas, an interesting section has recently been cut along the road up Borrowdale a little south of the Bowder Stone. Several bands of coarse amygdaloidal lava may there be seen interstratified among tuffs. The calcite amygdales in these rocks are arranged parallel to the bedding and therefore in the planes of flow, while those lined with chlorite are more usually deformed parallel to the direction of the cleavage. This difference suggests that before the cleavage took place, not improbably during the volcanic period, the rocks had been traversed by heated water producing internal alteration and rearrangements, in virtue of which the vesicles along certain paths of permeation were filled up with calcite, so as then to offer some resistance to the cleavage, while those which remained empty, or which had been merely lined with infiltrated substance, were flattened and pulled out of shape. Messrs. Harker and Marr have shown that the amygdaloidal kernels had already been introduced into the cellular lavas before the intrusion of the Shap granite. In the account to be given of the Tertiary plateau-basalts (Chapter xxxvi.) evidence will be adduced that this filling up of the steam-cavities of lava may take place during a volcanic period, and that it is probably connected with the passage of heated vapours or water through the rocks.

Though acid lavas are not wholly absent from the central and lower parts of the volcanic group, it is at the top that their chief development appears to occur. These rocks may be grouped together as felsites or rhyolites. They probably play a much larger part in the structure of the southern part of the volcanic area than the published maps would suggest, and a detailed survey and petrographical study of them would well reward the needful labour.[249]A fine series of felsites is interbedded in the lowerpart of the Coniston Limestone, and spreads out underneath it along the southern margin of the volcanic district from the Shap granite south-westward for some miles[250](Fig. 62). Between the valleys of the Sprint and Kent these felsites (which farther east are said to be 700 feet thick) may be seen interposed between the limestone and the fossiliferous calcareous shales below it, while from underneath the latter other sheets rise up into the range of hills behind.

[249]See Mr. F. Rutley, "The Felsitic Lavas of England and Wales,"Mem. Geol. Surv.1885, pp. 12-15; also the description of Messrs. Harker and Marr,Quart. Journ. Geol. Soc.xlvii. (1891), p. 301.[250]Unfortunately these acid lavas are not distinguished from the others in the Geological Survey maps.

[249]See Mr. F. Rutley, "The Felsitic Lavas of England and Wales,"Mem. Geol. Surv.1885, pp. 12-15; also the description of Messrs. Harker and Marr,Quart. Journ. Geol. Soc.xlvii. (1891), p. 301.

[250]Unfortunately these acid lavas are not distinguished from the others in the Geological Survey maps.

Fig. 62.—Section of felsites on the Coniston Limestone group, west of Stockdale.a, Felsites more or less cleaved;b, Calcareous shales with fossils, much cleaved;c, Cleaved felsite;d, Coniston Limestone;e, Stockdale Shales (with graptolites).

These acid lavas are generally grey, cream-coloured, or pink, with a white weathered crust. Their texture when fresh is flinty or horny, or at least extremely fine-grained and compact. They are seldom markedly porphyritic. They frequently display good flow-structure, and sometimes split up readily along the planes of flow. Occasionally the flow-lines on the outer crust have broken up in the movement of the rock, giving rise to irregular fragments which have been carried forward. Short, extremely irregular, branching veins of a fine cherty felsitic substance, which occasionally shows a well-marked flow-structure parallel to the walls, traverse certain parts of a dark-grey felsite, near Brockstones, between the valleys of the Kent and Sprint.[251]Occasionally a distinct nodular structure may be observed in these acid lavas, sometimes minute, like an oolite, in other parts, as on Great Yarlside, presenting large rounded balls. This nodular structure is not confined to the lava-flows, but has been detected by Messrs. Harker and Marr in what appears to be an intrusive rock near Shap Wells. The microscopic characters of some of the Lake District rhyolites were described by Mr. Rutley, who found them to exhibit beautiful perlitic and spherulitic structures.[252]That such rocks as these were poured out in a vitreous condition, like obsidian or pitchstone, cannot be doubted. Chemical analysis shows that the Lake District rhyolites agree exactly with those of North Wales in their composition. They contain about 76 per cent of silica.[253]

[251]Compare the structure described by Mr. Harker from the Cross Fell inlier,Quart. Journ. Geol. Soc.xlvii. (1891), p. 518.[252]"Geology of Kendal," etc.,Mem. Geol. Survey, Sheet 98 N.E. 2nd edit. p. 9.[253]Messrs. Harker and Marr,op. cit.p. 302.

[251]Compare the structure described by Mr. Harker from the Cross Fell inlier,Quart. Journ. Geol. Soc.xlvii. (1891), p. 518.

[252]"Geology of Kendal," etc.,Mem. Geol. Survey, Sheet 98 N.E. 2nd edit. p. 9.

[253]Messrs. Harker and Marr,op. cit.p. 302.

The rhyolitic lavas have been seriously affected by the general cleavage of the region. In some places they have been so intensely cleaved as to become a kind of fissile slate, and there seems good reason to believe that inthis altered condition they have often been mistaken for tuffs. Where they assume a nodular structure, the nodules have sometimes been flattened and elongated in the direction of the prevalent cleavage.

The abundance and persistence of thoroughly acid lavas along the southern edge of the volcanic area where the youngest outflows are found, is a fact of much interest and importance in the history of the eruptions of this region. It harmonizes with the observations made in Wales, where in the Arenig, and less distinctly in the Bala group, a marked increase in acidity is noticeable in the later volcanic products. At the same time, as above mentioned, there is evidence also of the discharge of more basic materials towards the close of the eruptions, and even of the outflow of a lava approaching in character to basalt.

According to the Geological Survey maps, by far the largest part of the volcanic district consists of pyroclastic materials. When my lamented friend, the late Mr. Ward, was engaged in mapping the northern part of the district, which he did with so much enthusiasm, I had an opportunity of going over some of the ground with him, and of learning from him his ideas as to the nature and distribution of the rocks and the general structure of the region. I remember the difficulty I had in recognizing as tuff much of what he had mapped as such, and I felt that had I been myself required, without his experience of the ground, to map the rocks, I should probably have greatly enlarged the area coloured as lava, with a corresponding reduction of that coloured as tuff. A recent visit to the district has revived these doubts. It is quite true, as Mr. Ward maintains, that where the finer-grained tuffs have undergone some degree of induration or metamorphism, they can hardly, by any test in the field, be distinguished from compact lavas. He was himself quite aware of the objections that might be made to his mapping,[254]but the conclusions he reached had been deduced only after years of unremitting study in the field and with the microscope, and in the light of experience gained in other volcanic regions. Nevertheless I think that he has somewhat exaggerated the amount of fragmental material in the northern part of the Lake District, and that the mapping, so consistently and ably carried out by him, and followed by those members of the Survey who mapped the rest of the ground, led to similar over-representation there. Some portions of the so-called tuffs of the Keswick region are undoubtedly andesites; other parts in the southern tracts include intercalated bands of felsite as well as andesite.

[254]He says: "I shall be very much surprised if my mapping of many parts of the district be not severely criticized and found fault with by those who examine only one small area and do not take into consideration all the facts gathered together, during the course of several years, from every mountain flank and summit" (op. cit.p. 25). Mr. Hutchings has expressed his agreement with the opinions stated in the text. He likewise coincides in the belief that there are many of these Lake District volcanic rocks, regarding which it is impossible to decide whether they are lavas or ashes (Geol. Mag.1891, p. 544).

But even with this limitation, the pyroclastic material in the Lake District is undoubtedly very great in amount. It varies in texture from coarse breccia or agglomerate, with blocks measuring several yards across, to the most impalpable compacted volcanic dust. In the lower parts of thegroup some of the tuffs abound in blocks and chips of Skiddaw Slate. Some good examples of this kind may be seen in Borrowdale, below Falcon Crag and at the Quayfoot quarries. Where the tuff is largely made up of fragments of dark blue slate, it much resembles the slate-tuffs of Cader Idris. Some of the pieces of slate are six or eight inches long and are now placed parallel to the cleavage of the rock. Among the slate debris, however, felspar crystals and felsitic fragments may be observed. Bands of coarser and finer green tuff show very clearly the bedding in spite of the marked cleavage (Fig. 63).

Fig. 63.—Fine tuff with coarser bands near Quayfoot quarries, Borrowdale.The highly-inclined fine lines show the cleavage. The more gently dipping bands and lines mark the bedding.

But throughout the whole volcanic group the material of the tuff is chiefly of thoroughly volcanic origin, and its distribution appears to agree on the whole with that of the bedded lavas. In the older portions of the group it is probably mainly derived from andesitic rocks, though with an occasional intermingling of felsitic or rhyolitic detritus, while in the higher parts many of the tuffs are markedly rhyolitic. Among the lapilli minute crystals of felspar, broken or entire, may be detected with the microscope. Some of the ejected ash must have been an exceedingly fine dust. Compacted layers of such material form bands of green slates, which may occasionally be seen to consist of alternations of coarser and finer detritus, now and then false-bedded. Such tuffs bring vividly before the mind the intermittent explosions, varying a little in intensity, by which so much of the fabric of the Lake mountains was built up.

Breccias of varying coarseness are likewise abundant, composed of fragments of andesite and older tuffs in the central and lower parts of the volcanic group, and mainly of felsitic or rhyolitic detritus in the upper parts. Some of these rocks, wherein the blocks measure several yards across, are probably not far from the eruptive vents, as at Sourmilk Gill and below Honister Pass. Generally the stones are angular, but occasionally more or less rounded. Stratification can generally be detected among these fragmental rocks, but it is apt to be concealed or effaced by the cleavage, while it is further obscured by that widespread induration on which Mr. Ward has laid so much stress. The extreme state of comminution of the volcanic dust that went to form the tuffs has probably caused them to be more liable to metamorphism than the lavas.[255]

[255]The microscopic and chemical characters of the Ash-Slates of the Lake District have been investigated by Mr. Hutchings,Geol. Mag.1892, pp. 155, 218.

Little has yet been done in identifying any of the vents from which the vast mass of volcanic material in the Lake District was ejected. Mr.Ward believed that the diabase boss forming the Castle Head of Keswick marks the site of "one of the main volcanic centres of this particular district,"[256]whence the great lava sheets to the southward flowed out. There are obviously two groups of bosses on the northern side of the district, some of which may possibly mark the position of vents. A few of them are occupied by more basic, others by more acid rocks. It is not necessary to suppose that the andesitic lavas ascended only from the former and the felsites from the latter. While the felsites on the whole are younger than the more basic lavas, they may have been erupted from vents which had previously emitted andesites, so that the present plug may represent only the later and more acid protrusions.

[256]Op. cit.p. 70.

Besides the boss of Castle Head there are numerous smaller basic intrusions farther down the Derwent Valley on either side of Bassenthwaite Lake. Among these are the highly basic rocks forming the picrite on the east side of the Dash Beck and the dykes on Bassenthwaite Common. All these bosses, sills, and dykes rise through the Skiddaw Slates, but there is no positive proof that they belong to the Lower Silurian volcanic series; they may possibly be much later.

The most important and most interesting of all the intrusive masses of basic material is that which constitutes a large part of the eminence that culminates in Carrock Fell. The remarkable variations in the composition of this mass have been already referred to. Mr. Harker has shown that while the centre of the mass is a quartz-gabbro, it becomes progressively more basic towards the margin. Through the gabbro a mass of granophyre has subsequently made its way, and along the line of junction has incorporated into its own substance so much of the basic rock as to undergo a marked modification in its structure and composition. Whether these intruded bodies of basic and acid material have ascended in one of the old volcanic funnels and have been injected laterally in laccolitic fashion has not been ascertained. Mr. Harker, indeed, is rather inclined to refer the intrusions to a time not only later than the Borrowdale volcanoes, but later even than the terrestrial movements that subsequently affected the district and gave the rocks their present cleaved and faulted structures. Besides the gabbro and granophyre of this locality, igneous activity has manifested itself in the uprise of numerous later dykes and veins, intermediate to basic in composition. Some of these are glassy (tachylyte) and spherulitic or variolitic.[257]

[257]Mr. Harker,Quart. Journ. Geol. Soc.vol. l. (1894) p. 312, li. (1895) p. 125.Geol. Mag.1894, p. 551.

Throughout the Lake District a considerable number of bosses of more acid rocks rise through the Skiddaw Slates, and likewise through the volcanic group even up to its highest members. Some of these bosses may possibly indicate the site of volcanic vents. Two of them, which form conspicuous features on either side of the Vale of St. John, consist of microgranite, and rise like great plugs through the Skiddaw Slates, as well as through thebase of the volcanic group. The view of the more eastern hill, as seen from the west, is at once suggestive of a "neck." These masses measure roughly about a square mile each.

With the acid intrusions may possibly be associated some of the other masses of granophyre, microgranite and granite (felsite, felstone, quartz-felsite, syenitic granite, quartz-syenite, elvanite), which have long attracted attention in this region. The largest of these intrusions is the tract of granite which stretches from Eskdale down to near the sea-coast as a belt about eleven miles long and from one to three miles broad. Another large mass is the granophyre or "syenite" of Ennerdale. Numerous other intrusions of smaller dimensions have been mapped.

To what extent any of these eruptive masses were associated with the volcanic phenomena remains still to be worked out. There seems to be little doubt that a number of them must belong to a much later period. Mr. Harker has expressed his belief that the intrusion of some of these igneous rocks was intimately associated with the post-Silurian terrestrial movements of which cleavage is one of the memorials.[258]The Skiddaw granite, though it does not touch any part of the volcanic group, but is confined to the underlying Skiddaw Slates, was erupted after the cleavage of the district, which affects the volcanic as well as the sedimentary series. In other instances also, as in that of Carrock Fell, the intrusion seems to have been later than the disturbances of the crust.[259]The amount of metamorphism around some of the bosses of granite is considerable. That of the Skiddaw region has been well described by J. C. Ward,[260]while that of the volcanic group by the Shap granite has been carefully worked out by Mr. Harker and Mr. Marr.[261]

[258]Quart. Journ. Geol. Soc.vol. li. (1895), p. 144.[259]Op. cit.p. 126.[260]"Geology of Northern Part of the English Lake District,"Mem. Geol. Surv.1876, chap. iii. The metamorphism around the diorites and dolerites, and the granophyres and felsites, is described in the same chapter.

[258]Quart. Journ. Geol. Soc.vol. li. (1895), p. 144.

[259]Op. cit.p. 126.

[260]"Geology of Northern Part of the English Lake District,"Mem. Geol. Surv.1876, chap. iii. The metamorphism around the diorites and dolerites, and the granophyres and felsites, is described in the same chapter.

[261]Quart. Journ. Geol. Soc.xlvii. (1891) p. 266, xlix. (1893) p. 359.

The Shap granite comes through the very highest member of the volcanic series, and even alters the Upper Silurian strata. It must thus be of much younger date than the volcanic history of the Lake District. It presents some features in common with the granite bosses of the south of Scotland. Like these, it is later than Upper Silurian and older than Lower Carboniferous or Upper Old Red Sandstone time. Its protrusion may thus have been coeval with the great volcanic eruptions of the period of the Lower Old Red Sandstone. It will accordingly be again referred to in a later chapter.

It must be confessed that none of the large bosses of massive rocks, whether diabases, gabbros, felsites, granophyres, or granites, appear to afford any satisfactory proof of the position of the vents which supplied the lavas and tuffs of the Lake District. Nor can such a decided accumulation of the volcanic materials in certain directions be established as to indicate the quarters where the centres of eruption should be sought. On the contrary, the confused commingling of materials, and the comparative shortness of theoutcrop of the several sheets which have been traced, rather suggest that if any one great central volcano existed, its site must lie outside of the present volcanic district, or more probably, that many scattered vents threw out their lavas and ashes over no very wide area, but near enough to each other to allow their ejected materials to meet and mingle. The scene may have been rather of the type of the Phlegræan fields than of Etna and Vesuvius. If this surmise be true, we may expect yet to recognize little necks scattered over the volcanic district and marking the positions of some of these vanished cones.

What appears to have been one of these small vents stands near Grange at the mouth of Borrowdale, where I came upon it in 1890. In the little Comb Beck, the Skiddaw Slates are pierced by a mass of extremely coarse agglomerate, forming a rudely-circular boss. The slates are greatly disturbed along the edges of the boss, so much so, indeed, that it is in some places difficult to draw a line between them and the material of the agglomerate. That material is made up of angular blocks, varying in size up to three feet long, stuck in every position and angle in an intensely-indurated matrix formed apparently of comminuted debris like the stones. The blocks consist of a finely-stratified shale, which is now hardened into a kind of hornstone, with some felsitic fragments. I could see no slags or bombs of any kind. There is no trace of cleavage among the blocks, nor is the matrix itself sensibly cleaved. I believe this to be a small volcanic neck and not a "crush-conglomerate." It has been blown through the Skiddaw Slates, and is now filled up with the debris of these slates. Its formation seems to have taken place before the cleavage of the strata, and its firm position and great induration enabled it to resist the cleavage which has so powerfully affected the slates and many members of the volcanic group.

It was the opinion of my predecessor, Sir Andrew Ramsay, and likewise of Mr. Ward, that the Cumbrian volcanic action was mainly subærial. This opinion was founded chiefly on the fact that, save at the bottom and top of the series, there is no evidence of any interstratified sediment of non-volcanic kind. The absence of such interstratification may undoubtedly furnish a presumption in favour of this view, but, of course, it is by no means a proof. Better evidence is furnished by the unconformability already mentioned between the Coniston Limestone and the lavas on which it lies. Besides angular pieces of lava, probably derived from direct volcanic explosion, this limestone contains fragments of amygdaloidal andesite, and also rolled crystals of striated felspar.[262]These ingredients seem to indicate that some part of the volcanic group was above water when the Coniston Limestone was deposited.

[262]Messrs. Harker and Marr,Quart. Journ. Geol. Soc.vol. xlvii. (1891), p. 310.

The absence of interstratifications of ordinary non-volcanic sediment in the Borrowdale group might conceivably arise from the eruptions following each other so continuously on the sea-floor, and at so great a distance from land that no deposition of sand or mud from the outside could sensibly affect the accumulation of volcanic material. Certainly some miles to theeast at the Cross Fell inlier, as already mentioned, there is evidence of the alternation of tuffs with the sandy and muddy sediment of the sea-bottom. Here, at the outer confines of the volcanic district, the ejected materials evidently fell on the sea-floor, mingled there with ordinary sediment, and enclosed the same organic remains. The well-defined stratification of many of the fine tuffs is rather suggestive to my mind of subaqueous than of subærial accumulation. At the same time, there seems no reason why, here and there at least, the volcanic cones should not have risen above the water, though their materials would be washed down and spread out by the waves.

One of the most marked points of contrast between the Cumbrian and the Welsh volcanic districts is to be found in the great paucity of sills in the former region. A few sheets of diorite and diabase have been mapped, especially in the lower parts of the volcanic group and in the underlying Skiddaw Slates. On the other hand, dykes are in some parts of the district not unfrequent, and certainly play a much more prominent part here than they do in the Welsh volcanic districts. The majority of them consist of felsites, quartz-porphyries, diorites, and mica-traps. But there is reason to suspect that where they are crowded together near the granite, as around Shap Fells, they ought to be connected with the uprise of the post-Silurian granitic magma rather than with the history of the volcanic group.[263]If this series of dykes be eliminated, there remain comparatively few that can with any confidence be associated with the eruption of the Borrowdale rocks.

[263]For a description of the dykes around the Shap granite see the paper by Messrs. Harker and Marr,Quart. Journ. Geol. Soc.vol. xlvii. (1891), p. 285.

vii.UPPER SILURIAN (?) VOLCANOES OF GLOUCESTERSHIRE

A remarkable group of igneous materials has long been known to rise among the Silurian rocks of the Tortworth district at the north end of the Bristol coal-field. They were believed to be aqueous deposits in the Wernerian sense by Weaver.[264]Murchison regarded them as intrusive sheets;[265]Phillips looked on them as partly intrusive and partly interstratified.[266]They consist largely of coarsely-amygdaloidal basalts, some of which have been microscopically examined.[267]But their field-relations as well as their petrography have not yet been adequately determined. They are represented on the Geological Survey Map as forming a number of parallel bands in strata classed as Upper Llandovery. If, as seems probable, some of them are really interstratified, they form the youngest group of Silurian volcanic rocks in England, Scotland, or Wales.

[264]Trans. Geol. Soc.2nd ser. vol. i. (1819), pp. 324-334.[265]Silurian System(1839), p. 457.[266]Mem. Geol. Surv.vol. ii. part i. (1848), p. 194.[267]"Geology of East Somerset," etc., inMem. Geol. Surv.(1876), p. 210; descriptions by Mr. F. Rutley.

[264]Trans. Geol. Soc.2nd ser. vol. i. (1819), pp. 324-334.

[265]Silurian System(1839), p. 457.

[266]Mem. Geol. Surv.vol. ii. part i. (1848), p. 194.

[267]"Geology of East Somerset," etc., inMem. Geol. Surv.(1876), p. 210; descriptions by Mr. F. Rutley.

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