HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 525 



position. Having as a rule two or more essential constituents. In 

 structure massive, crystalline, felsitic or glassy, or in certain altered 

 forms, colloidal. 



The following shows the types of labels used in this collection : 



HORNBLENDE-BIOTITE GRANITE. 



Near Salt Lake City, Utah. 39,°95- 



Gift of G. K. Gilbert, 1887. 



PERIDOTITE: Dunite. 



Near Webster, Jackson County, North Caro- 

 lina. 39, T 3i- 

 Gift of W. A. H. Schreiber, 1887. 



QUARTZITE [Novaculite]. 



Leigh River, Victoria, Australia. 28,321. 



Australian Centennial Commissioners, 1876. 



The inclosing a name in brackets, as [Novaculite] in the last form, 

 indicates that it has gone out of use, or is a local or popular name of 

 little value and not generally accepted. 



I. — Aqueous rocks. 



A. — ROCKS FORMED AS CHEMICAL PRECIPITATES. 



This comparatively small though by no means unimportant group of 

 rocks comprises those substances which, having once been in a condition 

 of vapor or aqueous solution, have been deposited as rock masses either 

 by cooling, evaporation, by a diminution of pressure, or by direct chem- 

 ical precipitation. It also includes the simpler forms of those produced 

 by chemical changes in preexisting rocks. Water, when pure or 

 charged with more or less acid or alkaline material, and particularly 

 when acting under great pressure, is an almost universal solvent. Thus 

 heated alkaline waters permeating the rocks of the earth's crust at great 

 depths below the surface are enabled to dissolve from them various 

 mineral matters with which they come in contact. On coming to the 

 surface or flowing into crevices the pressure is diminished or evapora- 

 tion takes place and the water, no longer able to carry its load, deposits 

 it wholly or in part as vein material or a surface coating. In other cases 

 alkaline or acid water bearing mineral matters may in course of their 

 percolations be brought in contact with neutralizing solutions and these 

 dissolved materials be thus deposited by direct precipitation. In still 

 other instances a substance wholly or in part volatile may, when buried 

 at considerable depths below the surface, be subjected to such temper- 

 ature as shall cause it to assume a gaseous state and pass upwards 

 until a cooler stratum is reached where it is again deposited. In these 

 various ways were formed the rocks here shown. 



