342 Allen and Crenshaw — Sulphides of Zinc, 



of the metal of the sulphide in the liquid phase, the absolute 

 amount of the variation in most cases would be very small on 

 account of the virtual insolubility of most mineral sulphides. 

 Comparatively soluble sulphides like zinc sulphide are excep- 

 tional. 



A comprehensive knowledge of necessary or even sufficient 

 conditions of composition in mineral genesis is limited by the 

 meagerness of what we know of chemical affinities. In 

 practice, we must be content with the investigation of those 

 systems which by reason of their simplicity or the light they 

 are likely to throw on natural phenomena are especially prom- 

 ising. The method of procedure will depend materially on the 

 class of minerals we have to stucby ; e. g., if we have under 

 consideration a magmatic mineral, the rock of which it- is a 

 constituent is a partial guide to the composition of the solution 

 in which it crystallized, although it must not be forgotten that 

 important volatile and soluble constituents may have escaped 

 during the processes of rock formation. The best way to 

 get light on the genesis of such a mineral is doubtless to 

 study the behavior of systems made up of components, gener- 

 ally oxides, which we find in the rock. If, however, the 

 mineral in question is a vein mineral, we have no similar 

 indication of the composition of the original solution. Still, 

 all lines of evidence indicate that the solution must have been 

 aqueous and that the constituent elements of the mineral existed 

 usually in more complex forms than those in the magmatic 

 solutions. Thus the sulphur of a sulphide may have existed 

 originally as hydrogen sulphide, soluble sulphides, sulphates or 

 even more complex forms, while the metal doubtless occurred in 

 the form of some salt. Now, while we might obtain valuable 

 chemical information by the investigation of systems consisting 

 of sulphur and the various metals, it would not be very helpful 

 in the elucidation of geological processes. To get at the com- 

 position of the systems in which vein minerals have formed, 

 we must rely rather on our knowledge of the composition of 

 springs and mine waters and on our knowledge of general 

 chemistry. At the same time, important subsidiary informa- 

 tion may be obtained by the study of various other systems 

 adapted especially to the case in hand. 



The determination of the geological portion of the subject 

 of mineral genesis might be left entirely to the geologist ; but 

 it will be evident to anyone that there are decided advantages 

 in the study of these questions by a method of consultation 

 between the laboratory and the field worker — a method which 

 affords opportunities for mutual suggestion and criticism, and 

 promises safer conclusions. 



The necessary and sufficient conditions for the formation of 

 the sulphides of zinc, cadmium, and mercury which are stated 

 in these pages have been worked out with geological aid from 



