SPECIFIC GRAVITIES OF MINERALS IN THE ZONES. 185 



zone the alterations result in increase in specific gravity, are believed to 

 hold and to be of fundamental importance in the metamorphism of rocks." 

 This principle of the development of minerals of low specific gravity 

 near the surface and of high specific gravity at depth has a direct applica- 

 tion to the crystallization of magmas. From a magma of a given chemical 

 composition there can be little doubt that the greater the depth, and there- 

 fore the greater the pressure at which crystallization occurs, the higher the 

 average specific gravity of the rocks. In this treatise no attempt will be 

 made to work out the applications of the rule to individual minerals and 

 rocks, but one illustration in reference to minerals and one illustration in 

 reference to rocks may be cited. It is well known that in the lavas silica 

 frequently crystallizes in the form of tridymite (sp. gr. 2.28-2.33), but that 

 in the deep-seated igneous rocks quartz (sp. gr. 2.65) only is found. It is 

 believed that the explanation of this fact is that near the surface other 

 factors than pressure control the crystallization, and therefore that the less 

 heavy form of crystallized silica — tridymite — may be produced; and that in 

 the lower zone pressure is the determinative factor in the crystallization, 

 and therefore that the heavy form of crystallized silica — quartz — invariably 

 results. An illustration in reference to rocks is the presence or absence of 

 glass. Glass has a lower specific gravity than the equivalent crystallized 

 substance. It is well known that where magmas crystallize near the surface 

 glass is a frequent product, and that where magmas crystallize deep below 

 the surface glass is either very subordinate or absent altogether. While 

 other factors besides pressure enter into this result, it is believed that the 

 frequent presence of glass near the surface and the presence of dense 

 crystallized minerals in the equivalent- deeper-seated rocks crystallizing 

 from magmas is a very striking illustration of the truth of the principle of 

 the development of minerals and rocks of low density where the pressure 

 is small and of great density where the pressure is great. 



o The above conclusions as to the condensation of material at considerable depths has an important 

 bearing upon Reade's theory of mountain making. (Reade, T. Mellard, The origin of mountain 

 making, London, 1886. ) His explanation of the rise of mountains is that the volume of the thick 

 deposits of sediments increases as a consequence of the rise of the isogeotherms. I believe that possible 

 expansion due to this cause is more than compensated in the case of the sediments by the mechanical 

 bringing of the particles closer together as the result of pressure, in many instances to the practical 

 disappearance of the interspaces, and by the condensation of the material itself by the physical- 

 chemical changes above explained. The condensation also has a bearing upon estimates of crustal 

 shortening. In so far as condensation occurs, shortening of the outer crust of the earth may allow 

 accommodation to a nucleus of decreasing size without crustal corrugation. 



