3§ NEW YORK STATE MUSEUM 



to Rosiwal, is indirectly proportional to the average size of grain 

 of the rock and directly to the length of the selected system of lines. 



A further improvement of this method has been recently de- 

 scribed by Hirschwald. 1 It consists of a microscopic eyepiece in 

 the focus of which are placed two glass plates, one ruled with 

 a set of ordinates and the other with abscissas, the latter plate being 

 movable along the edge of the first by means of a screw turned 

 with the fingers. The microscope, when focused upon the section, 

 shows the two scales superposed upon the surface ; the movable or 

 horizontal scale is used to measure the intercepts of the mineral 

 particles. By readjusting the movable scale, the measurement may 

 be repeated until the area of view is covered. It is recommended 

 by Hirschwald that the measurements be taken at such intervals 

 as to cover the average grains by two or three readings, the number 

 depending on the size of the particles. 



The microscopic method of approximating the chemical compo- 

 sition is considered by Hirschwald to be preferable to chemical 

 analysis in some instances. Such is the case with sandstones that 

 contain decomposable ingredients and those of hard siliceous nature, 

 and it serves equally well to determine the amount of cement. 



There is need of much care in selecting the samples for micro- 

 scopic examination to insure that they represent a fair average of 

 the rock. It is also unsafe to depend on the evidence obtained 

 from a single section. As the area of a section is usually less than 

 a square inch, the minerals may not be present in it in the same 

 proportion as in the rock mass, especially if the grain be coarse. 

 Inaccurate results are often much worse than none, as illustrated by 

 the misinformation that is often circulated by quarry owners and 

 which sometimes originates from supposedly reliable sources. 



CHEMICAL ANALYSIS 



The making of a complete chemical analysis of a rock is a labor- 

 ious operation that requires special equipment and much chemical 

 knowledge and experience. It is also expensive. For ordinary 

 practical purposes, and when the stone is not limestone or quartzite 

 for use in metallurgy or chemical manufacture, such analysis is 

 not required. 



In the case of igneous rocks, it is quite important to determine 

 the water, carbon dioxide and sulphur. The water and carbon 



1 Hirschwald, J. Handbuch der Bautechnischen Gesteinspriifung, Berlin, 

 1912, p. 146-47, 167-72. 



