696 



DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 

 Average Specific Gravities of Certain Types. 



The average specific gravities of holocrystalline types have been calculated, 

 with results shown in the following accessory table. Most of the determinations 

 were taken from Osann's book: — 





Number 



of specimens 



averaged. 



Average specific 





gravity. 





58 



5 



11 



2 



13 



17 



18 



4 



6 



21 



3 

 4 



2 



2 

 2 



2 

 2 

 2 

 2 



2 

 2 



3 



2 

 2 

 2 



660 





740 





773 





805 





600 

 861 

 933 





948 





715 

 176 



862 





917 





884 



Source of Magmatic Heat. 



Before a genetic classification of igneous-rock bodies can be worthily under- 

 taken, attention must be given to the problem of magmatic heat. Needless to 

 say, its full discussion is impossible in this report, but a summary statement of 

 the matter as understood by the writer will make clearer the following chapters. 



The reader will recall various older attempts to account for volcanic heat ; 

 some by exothermic chemical reactions underground; others by assuming a 

 sufficient concentration of the heat produced in the crushing of rock during the 

 folding of mountain chains. Such suggestions have certain value as partial 

 explanations, but have so far failed to account for the largest part of the heat 

 contained in extrusive and intrusive bodies. At present, most geologists are 

 inclined to believe in the very oldest of the scientific explanations, namely, that 

 magmatic heat is chiefly a residual of the primary heat in a cooling planet. 



According to the Laplacian statement of the nebular hypothesis, the eartt 

 was originally a small star, incandescent at its surface and centrally very much 

 hotter than the hottest known lava. The crusting and continued cooling of 

 such a spheroid must give isothermal surfaces rather closely parallel to its own 

 surface. On this hypothesis the depth at which ordinary rock-matter is hot 

 enough to be eruptible, because molten, is approximately the same all around the 

 globe, and for a vast period of time only a few miles, or tens of miles, below the 

 surface. 



According to Chamberlin's statement of the planetesimal hypothesis, the 

 earth has been a dark body, cool enough to bear a water-ocean and living crea- 



