4 INTRODUCTION. 



are displayed before us among the crystals of the earth. A text- 

 book on Crystallography, Physics, or Celestial Mechanics, printed 

 in our printing-offices, would serve for the universe. The universe, 

 if open throughout to our explorations, would vastly expand our 

 knowledge, and science might have a more beautiful superstruct- 

 ure, but its basement-laws would be the same. 



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

 l|i immensity itself in its revelations of truth ; and science, though 



gathered from one small sphere, is the deciphered law of all 

 I spheres. 



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



before entering upon the study of the earth. It gives grandeur 



1 to science and dignity to man, and will help the geologist to 



• apprehend the loftier characteristics of the last of the geological 



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



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

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

 inseparably connected, and in all geological investigations they go 

 together. Geology had its very beginning and essence in 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, clay -rock, and limestone, lying 

 above one another in many successions ; 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, therefore, they belong 

 to successive periods in the earth's past ; that, consequently, the 

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

 further, that each rock indicates some facts respecting the condi- 

 tion 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, — indeed, as actual historical records ; and every new fact 

 ascertained by a close study of their structure, be it but the occur- 

 rence of a pebble, or a seam of coal, or a bed of ore, or a crack, or 

 any marking whatever, is an addition to the records, to be inter- 

 preted 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 



