[426]Trans. Roy. Soc. Edin.vol. xxxvii. p. 125.
[426]Trans. Roy. Soc. Edin.vol. xxxvii. p. 125.
The neck of North Berwick Law was found by Dr. Hatch to be a trachyte, showing a plexus of lath-shaped sanidines that diminish in size to minute microlites, but with no porphyritic or ferro-magnesian constituent. The Bass Rock, though its geological relations are concealed by the sea, is in all probability another neck of this district. It is likewise a mass of trachyte, composed almost entirely of lath-shaped crystals of sanidine, with no ferro-magnesian constituent, but a good deal of iron ore. It shows none of the large porphyritic felspars so characteristic of the Garleton Hills lavas, but it closely resembles the non-porphyritic varieties, particularly the lavas of Score Hill, Pencraig, Lock Pit Hill, and Craigie Hill.[427]
[427]The composition of the rocks of North Berwick Law and the Bass closely resembles that of the trachytic lavas of the plateau. For analyses, see Dr. Hatch's Paper,ibid.pp. 123, 124.
[427]The composition of the rocks of North Berwick Law and the Bass closely resembles that of the trachytic lavas of the plateau. For analyses, see Dr. Hatch's Paper,ibid.pp. 123, 124.
(f)Tuffs.—The fragmentary ejections of the plateaux vary in texture from the finest-grained tuffs to coarse agglomerates.[428]As they have been derived from the explosion of andesite-lavas, they consist mainly of the debris of these rocks. They are often deep red in colour, as for example those of Dunbar, but are most frequently greenish. They have a granular texture, due to the small lapilli of various porphyrites imbedded in a fine dust of the same material. Grains of quartz, frequently to be detected even in the finer tuffs, may either have been ejected from the volcanic vents, or may have been grains of sand in the ordinary sediment of the sea-bottom. Both at the base and at the top of the plateau-series, the tuffs are interstratified with and blend into sandstones and shales, so that specimens may be collected showing a gradual passage from volcanic into non-volcanic detritus. In many of the tuffs of the necks fragments of sandstone andother stratified rocks occur, representing the strata through which the vents were drilled. In the tuffs of the Eaglesham district pieces of grey and pink granite have been met with which, if they are portions of an old granite mass below, must have come from a great depth.[429]In the coarser tuffs and agglomerates a larger variety of lava-form rocks is to be found than can be seen among the bedded lavas of the Plateaux. They include felsites and quartz-porphyries, and more rarely basic lavas (diabases, etc.).
[428]For accounts of these rocks, see Explanation of Sheet 33Geol. Surv. Scot.p. 32; Sheet 22, pp. 11-14; Sheet 31, pp. 14-17.
[428]For accounts of these rocks, see Explanation of Sheet 33Geol. Surv. Scot.p. 32; Sheet 22, pp. 11-14; Sheet 31, pp. 14-17.
[429]Explanation of Sheet 22Geol. Surv. Scot.p. 12.
[429]Explanation of Sheet 22Geol. Surv. Scot.p. 12.