IV.Post-tertiary or Quaternary—Historical or Recent.Pleistocene.III.Tertiary or Cainozoic—Pliocene.Miocene.Eocene.II.Secondary or Mesozoic—Cretaceous.Jurassic.Triassic.I.Primary or Palæozoic—Permian.Carboniferous.Devonian and Old Red Sandstone.Silurian.Cambrian.Laurentian or Pre-Cambrian.
95. ThePrimaryformations are so called because they are the oldest known to us: they are not necessarily the first-formed aqueous deposits. Dr Hutton said truly: There is no trace of a beginning, and no signs of an end. In thePrimaryorPalæozoic(ancient-life) formations are found the earliest traces of life. The forms as a rule depart very widely from those with which we are acquainted now. TheLaurentianrocks have yielded only one fossil—a large foraminifer namedEozoon Canadense. TheCambrianformation contains but few fossils—crustaceans, molluscs, zoophytes, and worm-tracks. TheSilurianstrata are often abundantly fossiliferous. All the great classes of invertebrates are represented, and fish remains also occur. TheDevonianandOld Red Sandstoneare also characterised by the presence of an abundant fauna. In the Old Red Sandstone are numerous fish remains; it appears to have been an estuarine or lacustrine deposit; the Devonian, on the other hand, was marine, like the Silurian and Cambrian. TheCarboniferousformation is the chief repository of coal in Britain. It consists of terrestrial, fresh or brackish water, and marine deposits. The fauna and flora of thePermian, which is partly a marine and partly a fresh-water formation, are allied, upon the whole, to those of the Carboniferous, but offer at the same time many contrasts.
96. TheSecondaryorMesozoic(middle-life) formations contain assemblages of fossils which do not depart so widely from analogous living forms as those belonging to Palæozoictimes. TheTriassicstrata yield abundance of rock-salt. In Britain they contain very few fossils, but these are more abundant in the Triassic deposits of foreign countries. The oldest known mammals first appear in this formation. TheJurassicformation is very highly fossiliferous. It is distinguished by the occurrence of numerous reptilian remains. Nearly all the beds of this formation are marine, but there are associated with these the remains of a forest or old land surface, and a considerable accumulation of estuarine or fresh-water deposits; impure coals also occur in this formation. TheCretaceousstrata are almost wholly marine, and chiefly of deep-water origin. But some land-plants are found, chiefly ferns, conifers, and cycads. Near the base of the formation occurs a great river deposit (Weald clay) with numerous remains of reptiles.
97. Among the oldest strata of theTertiaryorCainozoic(recent-life) division we meet with thedawnof the existing state of the testaceous fauna—theEocene(ēos, dawn, andkainos, recent) containing three and a half per cent. of recent species among its shells. The proportion of recent species increases in theMiocene(meion, less, andkainos, recent), although the majority of the molluscs entombed in that formation belong to extinct species. In thePliocene(pleion, more, andkainos, recent), however, the extinct species are in a minority.
ThePost-tertiaryorQuaternarydivision comprises the concluding chapters of geological history. ThePleistocene(pleistos, most, andkainos, recent) contains no extinct species of shells, but a number of extinct mammalia. In theRecentdeposits all the species of animals and plants are living. The Tertiary and Quaternary formations are partly of marine and partly of terrestrial and fresh-water origin. At the close of the Tertiary period the 'glacial epoch' of Pleistocene times began, and the British Islands and a large part of northern Europe and North America were then cased in snow and ice. Traces of glacial conditions have also been met with in the Eocene and Miocene. The evidence furnished by Palæozoic andMesozoic formations points chiefly to mild, genial, and sometimes tropical conditions. But traces of ice action are occasionally noted (namely, in the Silurian, Old Red Sandstone, Carboniferous, Permian, and Cretaceous formations), pointing, perhaps, in some of the cases, to former alternations of cold and warm periods. Indeed, the belief is now gaining ground, that the so-called glacial epoch of Pleistocene times was not one long continuous age of ice, but rather consisted of an alternation of warm and cold periods. And it is not improbable, but highly likely, that similar alternations of climate have happened during every period of great eccentricity of the earth's orbit.
QUESTIONS.
Section 1. What is Geology?
2. Define the termrock. How many classes of rock are there?
3, 4, 5. Into what groups are the mechanically formed rocks divided? Define the termsconglomerate,sandstone, andshale.
6. What is the nature of the rocks belonging to the Aërial or Eolian group?
7. Give an example of a chemically formed rock.
8. Give examples of organically derived rocks.
9. What kinds of rocks are embraced by the Metamorphic class?
10. What are igneous rocks?
12. What is the mineralogical composition of granite?
13. What is meant by amineral?
14. Name five minerals which do not contain oxygen. Where doesfluor-sparoccur? What is the element that enters most largely into the composition of the earth's crust?
15. Name the forms under which the mineralquartzoccurs. Name some of the oxides of iron. What isiron pyrites?
16. Name twosulphates. Name twocarbonates. Name some of thesilicates. In what kinds of rock isaugitefound? Where does it never occur? In what kinds of rock doeshornblendeusually occur? Mention three species of felspar. What is one of the most distinguishing characteristics of mica? Name three silicates of magnesia. Mention some of their distinguishing peculiarities. Where dozeolitescommonly occur?
17. What is aquartzose conglomerate? What is acalcareous conglomerate?
18. What isgrit? What isfreestone? To what are the various colours of sandstone due? What isshale?
19. Name some typical Eolian rocks, and tell where they occur.
20. How dostalactitesandstalagmitesoccur? What issiliceous sinter, and how does it occur? How doesrock-saltoccur?
21. Mention some of the varieties of limestone. What iscornstone? What is the composition ofdolomite?
22. Name some of the varieties of coal.
23. What isquartzite?
24. Describeclay-slate.
25. Mention some altered limestones.
26. What areschists? Name and give the mineralogical composition of three schists.
27. What is the general character of metamorphic rocks?
28. How would you classify granite?
29. What is the mineralogical composition ofsyeniteanddiorite?
30. How do we distinguish the two groups into which igneous rocks are subdivided? What is meant by the termsamygdaloidalandporphyritic?
31. Name some rocks that belong to the acidic group. What isquartz-porphyry?
32. Give examples of augitic igneous rocks. Name a hornblendic igneous rock.
33. What are fragmental igneous rocks? What is the difference betweentrappean brecciaandtrappean conglomerate?
34. What is meant by the termsstratum,strata, andstratified? What is the difference betweenlaminationandbedding? What is a section?
35. What isfalse bedding?
36. Briefly describe the general appearance ofmud-cracksandrain-prints, and say how these have been formed.
37. What is meant by asuccession of strata?
38. Which kinds of stratified rocks generally have the greatest extension?
39. How do beds terminate?
40. How may planes of bedding sometimes indicate a break in the succession of strata?
41. What is the nature ofjoints? What aremaster-joints, and what is their probable cause?
42. What iscleavage, and what is its effect upon the bedding of rocks?
43. What isfoliation?
44. Give examples of concretionary rocks. What is the nature of chert and flint nodules?
45. Define the termsdipandstrike. What is thecropof a bed? What areanticlinesandsynclines?
46. What is meant by aninversion of strata?
47. How does contemporaneous erosion indicate a pause in the deposition of a series of strata?
48. What is meant byunconformability? How does unconformability prove a lapse of time between the accumulation of the underlying and overlying strata?
49. What isoverlap?
50. What is afault? What ishade? How are the strata affected on either side of a fault? What is the appearance calledslickensides? Under what circumstances should we term a fault adownthrow? and when should we term it anupcast? How is the approximate age of a fault sometimes shewn?
51. What are metamorphic rocks, and what is their general appearance? In what districts of the British Islands are they most abundantly developed? What are some of the appearances relied upon for distinguishing metamorphic from igneous granite?
52. How do igneous rocks occur? Define what is meant bycontemporaneousandsubsequentorintrusiveigneous rocks. How does a contemporaneous igneous rock affect the beds upon which it rests? What is the character of the bed overlying a contemporaneous rock? What is the general structure of a contemporaneous igneous rock? What is meant byvesicular structure? What is the general texture of a contemporaneous igneous rock? What is the nature of the jointing in igneous rocks? What iswacké?
53. What is the nature of the beds ofbreccia,conglomerate,ash, andtuff, with which contemporaneous igneous rocks are often associated? What is aneckofvolcanic agglomerate? How are the strata affected at their junction with a 'neck'?
54. How do intrusive igneous rocks occur? How do intrusivesheetsoccur? What effect have they produced upon the strata above and below them? What is adyke? What relation do they occasionally bear tosheetsof igneous rock? What is aneckof intrusive igneous rock, and how have the strata surrounding it been affected?
55. Mention some of the contrasts betweenintrusiveand contemporaneous igneous rocks. What alteration is produced upon coal with which an intrusive sheet has come in contact?
56. What aremineral veins? What is the nature of the quartz veins in granite? How are the minerals usually arranged in the great metalliferous veins? What is apipe-vein?
57. What are the great geological agents of change?
58. What is meant byweathering? How are rocks affected at the surface in tropical countries? What chemical effect has the atmosphere on calcareous rocks? How is soil formed? How are sand dunes formed? Mention some effects of the transporting power of the atmosphere.
59. Mention some of the chemical effects of interstitial water. What is the origin oftravertineorcalcareous tufa?
60. How havestalactitesandstalagmitesbeen formed? Give some instances of the solvent power of springs.
61. How are caves in limestone formed? Describe some of the appearances of a country composed of calcareous rocks. Describe briefly how a river erodes its channel.
62. Describe the geological action of rain.
63. What do chemical analyses of river-water prove? Give an example. What are pot-holes? Give an example of the erosive power of running water. What amount of mud is carried in suspension by the Mississippi, and discharged annually into the sea? What estimate has been formed of the total amount of mineral matter annually transported by that river?
64. What isalluvium? How is it formed? and mention some examples of its occurrence.
65. How is sediment deposited by a river in a lake?
66. What is the difference between lacustrine and fluvio-marine deposits? What is adelta?
67. Describe the geological action of frost. Describe the geological action of river-ice.
68. What areglaciers? What thickness do they attain in the Alps? What is their rate of motion? What arecrevasses, and how do they originate? What aresuperficial moraines? What areterminal moraines? What changes does a glacier effect upon its bed, and how are these modifications produced? What is the character of a glacial river? What is the origin oficebergs? How is the general absence of blocks and stones in Greenland icebergs to be explained? What is the nature of a submarine terminal moraine? What is theice-foot? What is the chief agent in distributing erratic stones and blocks over the sea-bottom? What effect upon the sea-bed must stranding icebergs produce?
69. What are some of the chemical compounds held in solution in sea-water? Which of these go to form the shells and skeletons of marine animals?
70. Describe the action of breakers on a sea-coast. How does frost aid the wasting action of breakers? What effect has the nature of the rocks in the production of inequalities in a coast-line? Upon what part of the sea-bottom does the material derived by the action of the breakers chiefly accumulate? What effect have the tides and ocean currents in the distribution of sediment?
71. What is the general rule as regards fine-grained and coarse-grained deposits? Mention a partial exception to this rule. What effect have tidal currents in shallow seas?
72. How are rocks disintegrated through the action of plants? What is peat? What may be inferred from the occurrence of shell-marl underneath peat? What does the appearance of roots and trunks of trees, and of remains of land animals under peat, indicate?
73. What, generally, is the geological action of animal life?
74. What is coral? What is afringingreef? What is the general character of abarrierreef? Give an example of one. What is anatoll? What is the nature of coral rock? What is Mr Darwin's theory of the formation of coral reefs?
75. What is the nature of the Atlantic ooze? In what respects may it eventually come to resemble chalk and limestone? Mention an instance of the abundant occurrence in the sea of animalcules with siliceous coverings and skeletons. What is the nature of thered clay found at great depths in the Atlantic and Southern Oceans?
76. What are some of the notions held in regard to the internal condition of the earth? At what (average) rate does the temperature of the earth's crust increase as we descend from the surface?
77. What is the nature of the movements to which the earth's crust is subjected?
78. Describe the hypotheses advanced to account for earthquakes. Mention some of the effects of earthquakes—1st, as regards alterations of level; and 2d, as regards modifications of the surface.
79. Mention a good example of tranquil elevation and depression of the earth's crust. Mention some of the proofs of an elevatory movement. Give proofs that shew depression of the land. How may certain former changes of sea-level be accounted for without inferring any movement of the land?
80. What effect mustdepressionhave upon the strata forming the earth's crust? What is the result of a movement of elevation? What is the cause ofcleavage?
81. What is the nature of the materials thrown out during volcanic eruptions? What is the general structure of a volcanic cone? How does molten rock make its escape from the orifice of eruption? What is the meaning of the termslapillo,puzzolana, andceneri?
82. What is lava? Describe the general appearance and mode of progression of a stream of lava. What effect is produced upon fragments of rock caught up and inclosed in lava; and what changes are caused in the pavement upon which it cools? How does a lava stream entering a lake or the sea behave in regard to the sediment gathering therein? To what is the basaltic structure due? How are the axes of the prisms in a columnar igneous rock arranged? Name some of the varieties of lava. What is the origin of the vesicular structure in igneous rocks? What portions of a bed of lava are most frequently scoriaceous? In what kinds of lava is the vesicular structure most abundantly met with? How have the vesicles become flattened? In what manner have they been filled with mineral matter? What is the origin of the dykes of modern volcanic districts?
83. How is metamorphism on the large scale supposed to havebeen induced? How may granite be at one and the same time a metamorphic and igneous rock?
84. Mention some of the views held with regard to the origin of mineral veins.
85. What isdenudation? How do inclined strata prove that the strata have been denuded? How dofaultsafford proof of denudation? What have been the general effects produced by denudation on the face of the land?
86. What part have the subterranean forces acted in the formation of mountains? To what geological action is the present aspect of these mountains due? What has determined the direction of river valleys? How have the valleys, dells, &c. been formed? What effect have faults had in determining the direction of river valleys? What is supposed to be the origin of the deep rock-basins occupied by many fresh-water lakes? How is the waste of land by denudation compensated?
87. What arefossils? What is meant bypetrifaction? In what kind of rocks do fossils occur most abundantly, and in the best state of preservation? and what reason can be given for this?
88. How do fossils afford proof of varied physical conditions? Give a reason for some rocks being more barren of fossils than others.
89. State some of the characters which distinguish broadly the older fossiliferous strata from those similar accumulations which are being formed in our own day.
90. How may we identify formations in separate districts? How is the interrupted and partial distribution of strata to be accounted for?
91. In what respect do the fossils in younger strata differ from those in older strata? What general proof can be adduced to shew that species have become gradually extinct?
92. Give an instance how fossils prove changes of climate in the past. What is supposed to be the cause of great cosmical changes of climate? Describe Mr Croll's theory of cosmical changes of climate.
93. What is the test ofsuperposition? Mention another test of the relative age of strata.
94. Name the four great divisions under which the fossiliferous rocks are arranged.
95. Name the Primary or Palæozoic formations. What are the principal kinds of fossils found in the Old Red Sandstone? Which formation is the chief repository of coal in Britain?
96. In what other formations do coals occur? In which formation do the oldest known mammals occur? Name the Secondary formations.
97. Name the Tertiary formations. What kind of climate characterised the northern hemisphere at the beginning of Pleistocene times? What kinds of climate would appear from the evidence to have chiefly prevailed in Primary, Secondary, and Tertiary ages? Have we any trace of frigid conditions during these ages? What is the growing opinion with regard to the climatic conditions during the glacial period of Pleistocene times?
THE END.
Edinburgh: Printed by W. & R. Chambers.
FOOTNOTES:[A]There are various kinds of felspar; the one referred to above isorthoclase, or potash-felspar.[B]It is needless to describe the minerals minutely here. The student can only learn to distinguish the different species by carefully examining actual specimens.[C]Petros, a rock, andlogos, a discourse. Some geologists restrict this term to the study of thestructureandarrangement of rock-masses, and apply the termlithology(lithos, a stone, andlogos, a discourse) to the study of themineralogical composition of rocks.[D]The degree of inclination is very variable. It may occur at almost any angle up to vertical. But, as a rule, the hade of the more powerful faults is steeper than that of minor displacements.[E]Igneous rocks have also in some cases undergone considerable alteration; fine-grained tuffs, for example, occasionally assume a crystalline texture.[F]Palaios, ancient,onta, beings, andlogos, a discourse.[G]To this there are some exceptions. Certain small foraminifers, for example, met with in some of the oldest formations, do not seem to differ from species which are still living. The genusLingula(Mollusca) has also come down from remotest ages, having outlived all its earlier associates.[H]This holds strictly true, however, only in regard to comparatively limited areas. The student must remember that strata occurring in widely separate regions of the earth, even although they contain very much the same assemblage of fossils, are not necessarily contemporaneous, in the strict meaning of the word; for thefaunaandflora(the animal and plant life) may have died out, and become replaced by new forms more rapidly in one place than another. The term 'contemporaneous,' therefore, is a very lax one, and may sometimes group together deposits which, for aught that we can tell, may really have been accumulated at widely separated times.[I]Apo, away from;hēlios, the sun.[J]Peri, round about or near by;hēlios, the sun.
FOOTNOTES:
[A]There are various kinds of felspar; the one referred to above isorthoclase, or potash-felspar.
[A]There are various kinds of felspar; the one referred to above isorthoclase, or potash-felspar.
[B]It is needless to describe the minerals minutely here. The student can only learn to distinguish the different species by carefully examining actual specimens.
[B]It is needless to describe the minerals minutely here. The student can only learn to distinguish the different species by carefully examining actual specimens.
[C]Petros, a rock, andlogos, a discourse. Some geologists restrict this term to the study of thestructureandarrangement of rock-masses, and apply the termlithology(lithos, a stone, andlogos, a discourse) to the study of themineralogical composition of rocks.
[C]Petros, a rock, andlogos, a discourse. Some geologists restrict this term to the study of thestructureandarrangement of rock-masses, and apply the termlithology(lithos, a stone, andlogos, a discourse) to the study of themineralogical composition of rocks.
[D]The degree of inclination is very variable. It may occur at almost any angle up to vertical. But, as a rule, the hade of the more powerful faults is steeper than that of minor displacements.
[D]The degree of inclination is very variable. It may occur at almost any angle up to vertical. But, as a rule, the hade of the more powerful faults is steeper than that of minor displacements.
[E]Igneous rocks have also in some cases undergone considerable alteration; fine-grained tuffs, for example, occasionally assume a crystalline texture.
[E]Igneous rocks have also in some cases undergone considerable alteration; fine-grained tuffs, for example, occasionally assume a crystalline texture.
[F]Palaios, ancient,onta, beings, andlogos, a discourse.
[F]Palaios, ancient,onta, beings, andlogos, a discourse.
[G]To this there are some exceptions. Certain small foraminifers, for example, met with in some of the oldest formations, do not seem to differ from species which are still living. The genusLingula(Mollusca) has also come down from remotest ages, having outlived all its earlier associates.
[G]To this there are some exceptions. Certain small foraminifers, for example, met with in some of the oldest formations, do not seem to differ from species which are still living. The genusLingula(Mollusca) has also come down from remotest ages, having outlived all its earlier associates.
[H]This holds strictly true, however, only in regard to comparatively limited areas. The student must remember that strata occurring in widely separate regions of the earth, even although they contain very much the same assemblage of fossils, are not necessarily contemporaneous, in the strict meaning of the word; for thefaunaandflora(the animal and plant life) may have died out, and become replaced by new forms more rapidly in one place than another. The term 'contemporaneous,' therefore, is a very lax one, and may sometimes group together deposits which, for aught that we can tell, may really have been accumulated at widely separated times.
[H]This holds strictly true, however, only in regard to comparatively limited areas. The student must remember that strata occurring in widely separate regions of the earth, even although they contain very much the same assemblage of fossils, are not necessarily contemporaneous, in the strict meaning of the word; for thefaunaandflora(the animal and plant life) may have died out, and become replaced by new forms more rapidly in one place than another. The term 'contemporaneous,' therefore, is a very lax one, and may sometimes group together deposits which, for aught that we can tell, may really have been accumulated at widely separated times.
[I]Apo, away from;hēlios, the sun.
[I]Apo, away from;hēlios, the sun.
[J]Peri, round about or near by;hēlios, the sun.
[J]Peri, round about or near by;hēlios, the sun.