226 L. V. Pirsson — Rise of Petrology as a Science. 



rocks until 1850, when Sorby used it for investigating a 

 calcareous grit. Oschatz, in Germany, also about this 

 time independently discovered the same method. A fur- 

 ther advance was made in melting the cement, floating off 

 the slice, and transferring it to a suitable object-glass 

 with cover, a process still employed by many; though 

 most operators now cement the first prepared surface of 

 the rock chip directly to the object-glass, and mount the 

 section without transferring it. 



Next came the use of machinery to save labor in grind- 

 ing, and another step was made in the introduction of the 

 saw, a circular disk of sheet iron whose edge was fur- 

 nished with embedded diamond dust. This makes it 

 possible to cut relatively thin slices with comparative 

 rapidity, but the final grinding which requires experience 

 and skill must still be done by hand. Carborundum has 

 also largely replaced emery. The skill and technique of 

 preparers has reached a point where sections of rocks of 

 the desired thinness (0-001 inch), and four or five inches 

 square have been exhibited. 



The Era of Petrography. 



In these earlier days of the science, as noted above, 

 great difficulty was at first experienced in the recognition 

 of the minerals as they were encountered in the study of 

 rocks under the microscope. At that time the chemical 

 composition and outward crystal form of minerals were 

 relatively much better known than their physical and, 

 especially, their optical properties and constants. Some 

 beginnings in this had been made by Brewster, Nicol, and 

 other physicists, and the mineralogists had commenced 

 to study minerals from this viewpoint. Especially 

 Des Cloiseaux had devoted himself to determining the 

 optical properties of many minerals, and the writer, 

 when a student in the laboratory of Rosenbusch in 1890, 

 well recalls the tribute that he paid to the work of 

 Des Cloiseaux for the aid which it had afforded him in his 

 earlier researches in petrography. 



The twenty years following the publication of the 

 texts of Rosenbusch and Zirkel may be characterized as 

 the era. of microscopical petrography. A distinction is 

 drawn here between the latter word and petrology, a 

 distinction often overlooked, for petrography means lit- 

 erally the description of rocks, whereas petrology denotes 



