642 CROSS, IDDINGS, PIRSSON, WASHINGTON 



It is to be remembered that these terms leucophyre and 

 melaphyre imply nothing as to the composition of the ground- 

 mass. They strictly indicate nothing but its color. 



III. The glasses — glassy rocks have been classified on a 

 basis of luster and texture as follows : 



1. Obsidian — vitreous rock of any color, usually black, often 

 red, less often brown and greenish. 



2. Pitchstone — resinous and less lustrous than obsidian, and 

 consequently lighter colored. 



3. Perlite — glassy rock with perlitic texture produced by 

 small spheroidal fractures. 



4. Pumice — highly vesicular glass, usually white or very 

 light-colored. 



Each of these varieties may be non-porphyritic or porphyritic. 

 The latter may be called 



Vitrophyre (Vogelsang, 1867) and may be qualified by mineral 

 terms indicating the character of the prominent phenocrysts, 

 yielding quartz-vitrophyre , feldspar-vitrophyre , etc. They may also 

 be called porphyritic obsidian, pitchstone, perlite, or pumice. 



PART III. METHODS OF CALCULATION. 



In order to obtain concordant results in all cases in the 

 determination of the kinds and amounts of standard minerals 

 that correspond to a magma of any given chemical composition, 

 a uniform method of calculation is necessary. This calculation 

 may be made either from the chemical analysis of the rock or 

 from the quantitative estimate of the minerals actually pres- 

 ent in it. 



The calculation of standard mi?ierals belonging to the salic 

 and femic groups, rather than that of the actual minerals which 

 may be present in the rock, is warranted not only by the fact of 

 the variable possibilities of crystallization in one magma, but 

 because of the difficulty of determining the quantity and chem- 

 ical character of the minerals actually present in many rocks. 

 It is further warranted because of the impossibility of determin- 

 ing the minerals in a great number of rocks in which they are 



