RELATIONS OF ROCK FLO WAGE TO MOUNTAIN MAKING. 925 



tion give a decrease in volume of at least 10 to 15 per cent for the majority 

 of the sediments. This is not believed to be an overestimate for the 

 average of the pelites, psephites, and limestones, but the decrease in 

 volume for the sandstones is probably not so great as this. It is therefore 

 certain that the decrease in A^olume in consequence of reactions controlled 

 by compression in the deep-seated zone is vastly greater than any expan- 

 sion which can result from rise in temperature. And it is certain that 

 from the top of the zone of anamorphism to the greatest depth to which 

 observation extends the reactions of metamorphism tend to produce min- 

 erals of higher specific gravity. However, the entire zone of observation 

 is very narrow as compared with the distance to the center of the earth. 

 If the law observed continues to great depth, it would follow that deeper 

 within the earth than observation extends minerals of specific gravity higher 

 than any known are produced. There is every reason to believe that the 

 density of~the most densely crystallized minerals with which we are familiar 

 is far from the maximum possible density. Hence, as pointed out on pages 

 364-365, deep within the earth, where the pressures are vastly gTeater 

 than any with which we are familiar, it is certainly possible, and perhaps 

 probable, that there exists a class of heavy minerals of complex molecules 

 with which we are entirely unfamiliar. 



But whether this speculation be well founded or not, there is every 

 reason to believe that during geological time there has been steady conden- 

 sation of the volume of the earth as a result of crystallization and recrys- 

 tallization. For eras at least the earth has steadily lost heat, and magma 

 has changed to crystallized rock, which has less volume. Probably also 

 the pressures within the earth have increased in amount in consequence of 

 decreasing rotation period, and increasing pressure promotes recrystalliza- 

 tion, with the formation of heavy minerals. It seems to me highly probable 

 that the decrease in A r olume due to the crystallization and recrystallization 

 of rocks, with the production of minerals of high specific gravity, is prob- 

 ably one of the most potent of the processes which have resulted in dimin- 

 ished volume." If this be so, it appears that the sum total of metamorphic 

 processes is subordinate to the general law of gravity which steadily demands 

 decreasing volume for the earth. Recognizing this law, I have elsewhere 

 held that it can not be maintained that masses of the earth locally rise 



«Van Hise, 0. R, Estimates ancf causes of crustal shortening: Jour. Geol., vol. 6, 1898, pp. 59-60. 



