The Students Elements of Geology | Page 8

Arguments hence derived as to the Nature of Plutonic Action. Hydrothermal Action, or the Influence of Steam and Gases in producing Metamorphism. Objections to the metamorphic Theory considered.



CHAPTER XXXIV.
METAMORPHIC ROCKS-- CONTINUED.
Definition of slaty Cleavage and Joints. Supposed Causes of these Structures. Crystalline Theory of Cleavage. Mechanical Theory of Cleavage. Condensation and Elongation of slate Rocks by lateral Pressure. Lamination of some volcanic Rocks due to Motion. Whether the Foliation of the crystalline Schists be usually parallel with the original Planes of Stratification. Examples in Norway and Scotland. Causes of Irregularity in the Planes of Foliation.



CHAPTER XXXV.
ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS.
Difficulty of ascertaining the Age of metamorphic Strata. Metamorphic Strata of Eocene date in the Alps of Switzerland and Savoy. Limestone and Shale of Carrara. Metamorphic Strata of older date than the Silurian and Cambrian Rocks. Order of Succession in metamorphic Rocks. Uniformity of mineral Character. Supposed Azoic Period. Connection between the Absence of Organic Remains and the Scarcity of calcareous Matter in metamorphic Rocks.



CHAPTER XXXVI.
MINERAL VEINS.
Different Kinds of mineral Veins. Ordinary metalliferous Veins or Lodes. Their frequent Coincidence with Faults. Proofs that they originated in Fissures in solid Rock. Veins shifting other Veins. Polishing of their Walls or "Slicken sides." Shells and Pebbles in Lodes. Evidence of the successive Enlargement and Reopening of veins. Examples in Cornwall and in Auvergne. Dimensions of Veins. Why some alternately swell out and contract. Filling of Lodes by Sublimation from below. Supposed relative Age of the precious Metals. Copper and lead Veins in Ireland older than Cornish Tin. Lead Vein in Lias, Glamorganshire. Gold in Russia, California, and Australia. Connection of hot Springs and mineral Veins.
INDEX.
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STUDENT'S ELEMENTS OF GEOLOGY.



CHAPTER I.
ON THE DIFFERENT CLASSES OF ROCKS.
Geology defined. Successive Formation of the Earth's Crust. Classification of Rocks according to their Origin and Age. Aqueous Rocks. Their Stratification and imbedded Fossils. Volcanic Rocks, with and without Cones and Craters. Plutonic Rocks, and their Relation to the Volcanic. Metamorphic Rocks, and their probable Origin. The term Primitive, why erroneously applied to the Crystalline Formations. Leading Division of the Work.
Of what materials is the earth composed, and in what manner are these materials arranged? These are the first inquiries with which Geology is occupied, a science which derives its name from the Greek ge, the earth, and logos, a discourse. Previously to experience we might have imagined that investigations of this kind would relate exclusively to the mineral kingdom, and to the various rocks, soils, and metals, which occur upon the surface of the earth, or at various depths beneath it. But, in pursuing such researches, we soon find ourselves led on to consider the successive changes which have taken place in the former state of the earth's surface and interior, and the causes which have given rise to these changes; and, what is still more singular and unexpected, we soon become engaged in researches into the history of the animate creation, or of the various tribes of animals and plants which have, at different periods of the past, inhabited the globe.
All are aware that the solid parts of the earth consist of distinct substances, such as clay, chalk, sand, limestone, coal, slate, granite, and the like; but previously to observation it is commonly imagined that all these had remained from the first in the state in which we now see them-- that they were created in their present form, and in their present position. The geologist soon comes to a different conclusion, discovering proofs that the external parts of the earth were not all produced in the beginning of things in the state in which we now behold them, nor in an instant of time. On the contrary, he can show that they have acquired their actual configuration and condition gradually, under a great variety of circumstances, and at successive periods, during each of which distinct races of living beings have flourished on the land and in the waters, the remains of these creatures still lying buried in the crust of the earth.
By the "earth's crust," is meant that small portion of the exterior of our planet which is accessible to human observation. It comprises not merely all of which the structure is laid open in mountain precipices, or in cliffs overhanging a river or the sea, or whatever the miner may reveal in artificial excavations; but the whole of that outer covering of the planet on which we are enabled to reason by observations made at or near the surface. These reasonings may extend to a depth of several miles, perhaps ten miles; and even then it may be said, that such a thickness is no more than 1/400 part of the distance from the surface to the centre. The remark is just: but although the dimensions of such a crust are, in truth,