In writing the last-mentioned work I had been called upon to pass in review almost all the leading points of speculation and controversy to which the rapid advance of the science had given rise, and when I proposed to bring out a new edition of the “Elements” I was strongly urged by my friends not to repeat these theoretical discussions, but to confine myself in the new treatise to those parts of the “Elements” which were most indispensable to a beginner. This was to revert, to a certain extent, to the original plan of the first edition; but I found, after omitting a great number of subjects, that the necessity of bringing up to the day those which remained, and adverting, however briefly, to new discoveries, made it most difficult to confine the proposed abridgment within moderate limits. Some chapters had to be entirely recast, some additional illustrations to be introduced, and figures of some organic remains to be replaced by new ones from specimens more perfect than those which had been at my command on former occasions. By these changes the work assumed a form so different from the sixth edition of the “Elements,” that I resolved to give it a new title and call it the “Student’s Elements of Geology.”
In executing this task I have found it very difficult to meet the requirements of those who are entirely ignorant of the science. It is only the adept who has already overcome the first steps as an observer, and is familiar with many of the technical terms, who can profit by a brief and concise manual. Beginners wish for a short and cheap book in which they may find a full explanation of the leading facts and principles of Geology. Their wants, I fear, somewhat resemble those of the old woman in New England, who asked a bookseller to supply her with “the cheapest Bible in the largest possible print.”
But notwithstanding the difficulty of reconciling brevity with the copiousness of illustration demanded by those who have not yet mastered the rudiments of the science, I have endeavoured to abridge the work in the manner above hinted at, so as to place it within the reach of many to whom it was before inaccessible.
CHARLES LYELL.
73 HARLEY STREET, LONDON,
December, 1870.
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.