GROWTH OF GEOLOGY. 271 



as the nature of rock-structure is known. In a few 

 cases the problem is easy of solution, as when it is 

 seen that some kinds of limestone are almost entirely 

 composed of the shells of Foraminif era ; in most 

 cases the problem is all unsolved. 



All that we can do in this section is to indicate 

 some of the important steps which have led to the 

 present vigorously progressive science of petrology 

 or petrography. 



Early Methods. In 1800 Fleurian d.e Bellevue 

 recommended the microscopic study of powdered 

 fragments of rock, and Cordier, in 1815, resorted to 

 this primitive device, and succeeded after much la- 

 bour in proving that basalt was made up of several 

 minerals. In the fourth decade of the century 

 Ehrenberg began to apply the microscope to minute 

 splinters and powdered fragments of various non- 

 crystalline rocks, and showed that some of these 

 were almost entirely composed of shells of minute 

 animals or plants, e.g., Foraminifera and Diatoms. 

 The step was important in itself and not less in its 

 suggestive value. 



About the middle of the century G. Bischof pub- 

 lished his text-book of chemical and physical geology 

 (1848-55), in which he compared the earth to " a 

 great chemical laboratory." Although he pushed 

 chemical interpretations to an extreme, he suggested 

 a point of view which in later days has seemed to 

 many like a Pisgah. From Bischof and Bunsen to 

 the scientists of to-day there is a long list. 



The Section Method. It is said that the first to 

 suggest and arrange the method of preparing thin 

 sections of rocks was William Nicol, the inventor 

 (1829) of the most useful prism of Iceland spar 

 that bears his name. A description of his method 



