74 THE POPULAR SCIENCE MONTHLY. 



ries an impressive lesson as to the unity prevailing amid all the 

 diversity of Nature, besides affording the hope that we may at 

 some time discover the origin of life, since it has already opened 

 the way to an explanation of the origin of the existing forms 

 of life ; while the grand outcome of geological study is that it 

 brings vividly before the mind the immensity of time, enabling 

 us to realize that time is only less than eternity. It also teaches 

 us that our earth has had a history, that our own race has had 

 a high antiquity ; and thus the contemplation of past geological 

 ages, reckoned by millions of years, the fact that our earth is 

 coeval with the sun in age all these considerations tend to im- 

 measurably expand our mental horizon, and thus to react in a 

 way to broaden the mind. 



Geology is also the complement of biology. As soon as one 

 has mastered the rudiments of botany and zoology, and of the 

 distribution of life-forms in space, the range of his thoughts 

 should be extended to take in the orderly succession of life in 

 past ages, and the evolution of modern specialized plants and 

 animals from the earlier, generalized types. No liberally edu- 

 cated person can, then, afford to ignore the study, and it seems to 

 us that it should be taken up for the following, among many 

 other considerations : 



1. Our first reason is that geology throws light on the origin 

 of our earth and of the solar system in general ; the facts and 

 speculations which culminated in the modern nebular hypothesis 

 give some idea of the steps by which our planet assumed its 

 present form and became adapted for the maintenance of life. 



2. After the earth cooled down and assumed its present shape 

 and size ; in some way unknown to us, monads and bacteria, to- 

 gether with infusoria, one-celled plants and animals, began to 

 exist, and geology hints that the period when all this became 

 possible may have been the early Laurentian, or at least at the 

 dawn of what, for a better name, we call archsean time. 



3. We now feel quite sure that the diversity of life of the 

 Cambrian period must have been in some way the result of 

 great changes in the physical geography of that time, and cor- 

 related with the inequalities of the sea-bottom, with regions of 

 shallows and of abysses, with landlocked areas, islands, and in- 

 cipient continents, rising from submarine plateaus bearing mount- 

 ain-chains. Geology describes the birth of continents, the rise 

 of mountain-chains, and discusses the results of the action of 

 heat in transforming the physical features of our globe, and 

 thus, in part at least, explains the origin of volcanoes, the causes 

 of earthquakes, and the processes of mountain-carving, through 

 the agency of brooks and rivers. 



4. Over immense tracts of mountainous regions, rocks, origi- 



