HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 



515 



Bridaepart 

 ss. Call- 



Auait&. Olume. 

 PlaqiaclasC' felspars 



FiR. 89. 

 Mounted thin suction 

 of Rock as Phepaued 

 foktiie Microscope. 



means of a solar camera, the final print being on glass and 1 foot in 

 diameter ; that is, that portion of the stone, which is in reality about 

 one-fourth of an inch in diameter, is here made to 

 appear 1 foot in diameter. These illustrations were 

 then painted by hand, the artist taking his colors 

 from an examination of the section itself under the 

 microscope. The eolors of the various minerals, it 

 will be observed, are not always the true colors of 

 the minerals themselves, but rather the color they 

 assume when, after being cut at different angles 

 with their optic and crystallographic axes, they are 

 viewed by means of polarized light. Such colors 

 are therefore somewhat misleading at first, but are 

 rendered necessary for the purposes of identification 

 and to bring out sharply the lines of separation be- 

 tween one mineral and another, and thus show the 

 structure and composition of the rock. Owing to 

 the thinness of the section (which is about -g^g- of an 

 inch) it would appear in ordinary light, i. e., light 

 not polarized, nearly colorless or with only dark flecks and faint tinges 

 of color here and there. 



This process of preparing thin transparent sections from rocks and 

 studying them by means of a compound microscope is of compara- 

 tively modern origin, naving come into general use only within the 

 past dozen years. Although the practice of grinding down thin sec. 

 tions of fossils was followed to some extent by H. Witham as early as 

 1831, the importance of its application to minerals and rocks does not 

 appear to have been fully realized until as late as 1838, when Dr. EI. (J. 

 Sorby announced the results obtained by him iu examining thin sec- 

 tions of simple minerals. Since this latter date the progress has been 

 steady and rapid, and has given a fresh impetus to geological research* 



The efficiency of the method is based upon the fact that every crys- 

 tallized mineral has certain definite optical properties, i. e., When cut 

 in such way as to allow the light to pass through it, will act upon this 

 light in a manner sufficiently characteristic to enable one working with 

 an instrument combining the properties of a microscope and stauro- 

 scope to ascertain at least to what crystalline system it belongs, and in 

 most cases by studying also the crystal outlines and lines of cleavage 

 the mineral species as well. To enter upon a detailed description of the 

 method by which this is done would be out of place here, since it in- 

 volves the subject of polarization of light and other subjects which 

 must be studied elsewhere. The reader is referred to any authoritative 

 work on the subject of light, and to Mr. J. P. Idding's translation of 

 Professor Rosen busch's work on optical mineralogy.* 



This method is of value, not merely as an aid in determining the min- 



* Microscopic Physiography of Rock-making minerals, Wiley &, Son, New York. 



