A RECORDING MICROMETER FOR GEOMETRICAL 

 ROCK ANALYSIS 



S. J. SHAND 

 Victoria College, Stellenbosch, South Africa 



The quantitative estimation of minerals in rock sections is 

 generally recommended as a valuable exercise, but in practice it is 

 far too seldom performed. The reason is that the recognized 

 methods of estimation are very tedious, while the results, when 

 obtained, have not hitherto been put to any systematic use in the 

 classification of rocks. The usual methods are of two kinds, viz., 

 (i) separations, either gravitational or magnetic, the separated 

 portions being weighed directly; and (2) geometrical methods, 

 involving measurement either of areas or of diameters, and sub- 

 sequent calculation of percentage volumes and percentage weights. 



Of estimations of the latter class, the following variants are 

 known to me: 



1. The method of Delesse: The surface of the rock is polished 

 and oiled, and the outlines of the grains are traced on transparent 

 paper, the areas corresponding to different minerals being distinc- 

 tively colored. The paper is then pasted upon tinfoil, and cut up 

 along the boundaries of the grains. The fragments having been 

 grouped according to color, the paper is removed and the tinfoil 

 weighed. The weights so found are proportional to the areas traced 

 upon the paper, hence also to the volumes occupied by the different 

 kinds of grains, provided that the rock is uniform throughout. To 

 get the proportions of the various minerals by weight, each volume 

 must be multiplied by the specific gravity of the corresponding 

 mineral. 



2. The outlines of the grains may be traced upon squared 

 paper, and the areas obtained by counting the number of squares 

 occupied by each mineral, all broken squares being reckoned as 

 half-squares. 



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