40 PHYSICAL AND CHEMICAL PROPERTIES OP ROCKS 



2S663 



Bridaeparf) 

 Calv 



dental air bubbles — and finally pressed flat down against the 



stone film. The film itself, if sufiiciently warmed, no longer ad- 

 heres to the thick glass, and may be removed 

 to the clean slip for its final mounting. This 

 is best accomplished by taking up the thick 

 glass by means of a pair of forceps and push- 

 ing eover-giass and film together, with a 

 needle point set in a handle, off into the 

 balsam on a new slide. The cover-glass here 

 serves merely as a support for the thin film 

 during the process of transferring. "Without 

 it there is danger of breakage. When fairly 

 transferred, the new slide is removed from 

 the hot plate, the cover pressed close down 

 against the film, adjusted in proper position 

 and allowed to cool. The superfluous balsam 

 may be then removed with a hot knife and 



the section finally washed in alcohol. Thus completed, it forms 



the ''thin section" of the petrologist 



Plagmdase. feldspars 



Fig. 2.-Mouiitecl thin 

 section of roek. 



2. THE SPECIFIC GRAVITY OP BOCKS 



The term specific gravity is used to designate the weight of 

 any substance when compared with an equal volume of distilled 

 water at a temperature of 4° C. This property is therefore 

 dependent upon the specific gravity of its various constituents 

 and their relative proportions. The exact or true specific 

 gravity of a rock may be obscured by its structure. Thus an 

 obsidian pumice will float upon water, buoyed up by the air 

 contained in its innumerable vesicles, while a compact obsidian 

 of precisely the same chemical composition will sink almost 

 instantly. This property of any subject is spoken of as the 

 apparent specific gravity in distinction from the actual com- 

 parative weight, bulk for bulk, of its constituent parts, which 

 could in the case of a pumice be obtained only by finely pul- 

 verizing so as to admit the water into all its pores. Inasmuch 

 as the structural peculiarities of any igneous rock — as will be 

 noted later — are dependent upon the condition utnder which it 

 cooled, it is instructive to notice that a crystalline aggregate 

 has a higher specific gravity, i e,, a greater weight, bulk for 



