A REVIEW OF THE AMORPHOUS MINERALS 541 



Glass, like amorphous substances in general, is in part optically 

 isotropic and in part birefringent. The birefringence is shown in 

 many perlites, some varieties of which are veritable Prince Rupert's 

 drops. Besides the chemical composition, the most important 

 property of glass is its refractive index, which, as has been shown by 

 Stark, 1 varies from about 1.485 for "acid" glasses with 75 per 

 cent silica up to 1.67 for "basic" glasses with 40 per cent silica. 

 Obsidian, perlite, pitchstone, etc., are petrographic terms, but 

 glass as a whole may be treated as a mineraloid from the mineral- 

 ogical standpoint. Lechatelierite and maskelynite are glasses 

 which are fairly definite in chemical composition. 



SUMMARY 



Some of the naturally occurring amorphous substances are 

 definite enough to be recognized as mineral species. 



The amorphous equivalents of crystalline minerals should be 

 treated as distinctive minerals and should have distinctive names. 



About twenty of the more prominent and well-defined amor- 

 phous minerals are described and discussed. Most of these minerals 

 are the amorphous equivalents of crystalline minerals. 



New names are given to amorphous cadmium sulfid (xantho- 

 chroite) and amorphous copper silicate (cornuite) which corre- 

 spond to sphalerite, greenockite, and chrysocolla respectively. 



Arguments are advanced for treating the natural hydrocarbons 

 and natural glasses as mineraloids. 



1 Min. u. Petr. Mitth., XXIII (1904), 536-50. 



