274 The American Naturalist. [April, 



Geologists who study the earth, not merely to satisfy their own 

 curiosity as to the present condition of things, but for the pur- 

 pose of advancing the science, and unraveling the mysteries of 

 the past, in order to produce a history of the planet as accu- 

 rately as human knowledge in its present condition will per- 

 mit, are only satisfied with the broad and comprehensive view. 

 Geology, by the aid of astronomy and physics, therefore, be- 

 gins with a great nebulous mass, of which all celestial bodies 

 were component parts. It traces the evolution of each body, 

 and that of the earth in particular. Starting when the earth 

 was thrown off as a ring of cloudy or gaseous elements, it 

 traces it through its transformation into a sphere of molten 

 matter surrounded with gases, through which the parent body, 

 the sun, could not penetrate. We learn of the war that existed 

 between the congealing surface aud the liquid interior in 

 which the former came off victorious, and formed a crust 

 through which the latter seldom broke. Then began the war 

 between the condensing vapors and the heated crust, in which 

 the latter succumbed to the overpowering element that fell 

 upon it and fairly covered it. 



Geology tells us of the life that existed in this mighty ocean 

 after it became sufficiently cooled, and in the powerful in- 

 ternal movements that resulted in the upheaval of masses of 

 rock that were to be the nuclei of the present continents, the 

 history and the formation of which is traced with great min- 

 uteness, and the life of each is described with great care, from 

 the lowest forms to the highest, and also the period in which 

 each form lived. 



There are several principles by which the geologist is guided 

 in answering the questions that continually arise as he studies 

 the earth with its many characteristics. 



1 In the first place, he understands that geology is an induc- 

 tive science. That is, it is a process of demonstration in which 

 a general truth is gathered from an examination of a self- 

 evident truth. Let me illustrate : From the study of modern 

 glaciers he learns certain facts in regard to conditions neces- 

 sary for their formation, their modes of action, and the results 



