4 INTRODUCTION. 



The earth, therefore, although but an atom in immensity, is im- 

 mensity itself in its revelations of truth ; and science, though gathered 

 from one small sphere, is the deciphered law of all spheres. 



It is well to have the mind deeply imbued with this thought, before 

 entering upon the study of the earth. It gives grandeur to science 

 and dignity to man, and will help the geologist to apprehend the loftier 

 characteristics of the last of the geological ages. 



Special aim of geology, and method of geological reasoning. — 

 Geology is sometimes defined as the science of the structure cf the 

 earth. But the ideas of structure and origin of structure are insepar- 

 ably connected, and in all geological investigations they go together. 

 Geology had its very beginning and essence hi the idea that rocks 

 were made through secondary causes ; and its great aim has ever been 

 to study structure in order to comprehend the earth's history. The 

 science, therefore, is a historical science. It finds strata of sandstone, 

 clayey rocks, and limestone, lying above one another in many succes- 

 sions ; and, observing them in their order, it assumes, not only that the 

 sandstones were made of sand by some slow process, clayey rocks of 

 clay, and so on, but that the strata were successively formed; that, there- 

 fore, they belong to successive periods in the earth's past ; that, con- 

 sequently, the lowest beds in a series were the earliest beds. It hence 

 infers, further, that each rock indicates some facts respecting the con- 

 dition of the sea or land at the time it was formed, one condition 

 originating sand deposits, another clay deposits, another lime, — and, 

 if the beds extend over thousands of square miles, that the several 

 conditions prevailed uniformly to this same extent at least. The rocks 

 are thus regarded as records of successive events in the history, — in-' 

 deed, as actual historical records ; and every new fact ascertained by 

 a close study of their structure, be it but the occurrence of a pebble, 

 or a seam of coal, or a bed of ore, or a crack, or any marking what- 

 ever, is an addition to the records, to be interpreted by careful study. 



Thus every rock marks an epoch in the history ; and groups of rocks, 

 periods ; and still larger groups, ages ; and so the ages which reach 

 through geological time are represented in order by the rocks that ex- 

 tend from the lowest to the uppermost of the series. 



If, now, the great beds of rock, instead of lying in even horizontal 

 layers, are much folded up, or lie inclined at various angles, or are 

 broken and dislocated through hundreds or thousands of feet in depth, 

 or are uplifted into mountains, they bear record of still other events 

 in the great history ; and should the geologist, by careful study, 

 learn how the great disturbance or fracture was produced, or succeed 

 in locating its time of occurrence among the epochs registered in the 

 rocks, he would have interpreted the record, and added not only a fact 



