50 NEW YORK STATE MUSEUM 



triangular shaped pieces by the slender, fibrous laths. The only- 

 similar occurrence that has been noted is the development of zeolitic 

 minerals between the thin laminae of graphite flakes in certain 

 contact zone rocks of the northern area. 



Microscopic Analyses 



In addition to the chemical analyses here quoted, a number of 

 microscopic analyses of the graphite ores of the different graphite 

 properties are included in this report. As this type of analyses and 

 the method of making it is unusual, a word concerning it is intro- 

 duced at this point. 



The microscopic analyses were made by means of a petrographic 

 microscope equipped with a camera lucida. By this arrangement 

 the outlines of the mineral grains in a given field were traced upon 

 coordinate paper and the areas occupied by the different species 

 determined by either one of two methods. For the larger areas a 

 polar planimeter was employed that measured the areas directly. 

 For the smaller units the areas were secured by counting the in- 

 dividual squares of the cross-section paper. These two methods 

 were used together as the paper was ruled in the same units as those 

 recorded by the planimeter. The sum of the areas occupied by the 

 grains of the different numerals was assumed to be proportional to 

 their volumes. By multiplying the volumes by the average specific 

 gravities of the minerals the proportion by weight was secured and 

 then calculated to ioo per cent. Usually three different microscopic 

 fields to each slide were analyzed and their results averaged. Care 

 was taken to use an optical system (objectives and oculars) so that 

 the largest practical field was obtained. The results of this method 

 are only approximate and should be regarded as merely indicating 

 the proper order of magnitude. In case of the graphite special 

 pains were taken to secure as accurate a result as possible. For 

 this mineral the probable error is likely about ± i per cent while for 

 the more abundant minerals an error of ±2 per cent to ±3 per cent 

 at least can be expected. It has not been possible up to the present 

 to check these results against a chemical analysis, for a given hand 

 specimen of this sedimentary schist is variable and any two speci- 

 mens, even if taken from the same spot, will show slight differences 

 in mineralogical make-up. 



The measurements made upon the graphite flakes were secured in 

 a similar manner, using the camera lucida. A scale, adapted for 

 the particular optical system employed, was moved into position 



