QUARRY MATERIALS OF NEW YORK 37 



perienced hands the method can be made to give the essential 

 features with sufficient accuracy for all practical purposes. 



The microscope used for rock examination is of special con- 

 struction, differing from the ordinary instrument chiefly in the use 

 of polarized light which is secured by two Nicol prisms, one of 

 which is placed below the stage and the other either in the tube or 

 above the eyepiece. 



Rock samples for examination under the microscope must be 

 reduced to such thinness that- they are perfectly transparent. This 

 means a thickness of o.i mm or less. The sections are prepared 

 from chips an inch or so in diameter that are broken off from 

 the rock sample with a small hammer, or better from flat pieces cut 

 with the diamond saw. These are ground smooth on one surface 

 with the aid of a lap wheel or glass plate, using emery or car- 

 borundum and water for abrasive. When a perfectly flat surface, 

 free of scratches, is obtained, this is cemented to the object glass 

 with Canada balsam. The other side is then ground down until 

 the section is of the required thinness, after which the sample is 

 cleaned and a cover glass cemented on it with balsam. The 

 preparation is permanent and can be filed away for future reference. 



To determine the proportions of the minerals in the section, from 

 which determination the chemical composition may be reckoned with 

 some degree of accuracy, the method adopted is that first devised 

 by Delesse 1 and later perfected by Rosiwal. 2 This depends upon 

 the principle that the areas occupied by the several minerals in the 

 section bear the same relations as the respective volumes of the 

 minerals. Delesse made a tracing of the outlines of the minerals, 

 gave each species a separate color, and then applied the tracing to 

 a sheet of tinfoil. The latter was divided carefully along the 

 boundaries of the minerals and the pieces corresponding to each 

 species were separately weighed. The result gave the proportions 

 of the several ingredients. The Rosiwal modification consists of 

 tracing on the cover glass a network of lines equally spaced and 

 intersecting each other at right angles. The ratio of the total length 

 of the lines to the sum of the intercepts of the mineral particles on 

 the lines is approximately the ratio of the total surface to the area 

 occupied by each mineral. The accuracy of the method, according 



1 Delesse, M. A. Procede mecanique pour determiner la composition des 

 roches. Paris, 1862. 



2 Rosiwal, August. Ueber geometrische Gesteinsanalysen, Verhandlungen 

 der K. K. geologischen Reichsanstalt zu Wien. v. 32, p. 143-75. 



