212 JOSEPH BARRELL 



TABLE V 



Rock Specific Gravity 



Granite 2 . 63-2 . 75 



Syenite 2.6-2.8 



Diorite 2.8-3.1 



Dolerite 3 • o -3 • 3 



Limestone 2.6-2.8 



Sandstone 2.5-2.7 



Shale 2.4 -2.8 



Slate About 2.8 



[The specific gravity of shale, although the most abundant of sedimentary 

 rocks, is not given in any of the manuals of geology, but Professor Hobbs, who 

 has read much of this manuscript and to whom the writer is indebted for a 

 number of suggestions, has called attention to the above figure as given by 

 Trautwine. In general, Trautwine and Kent give a somewhat greater range 

 in specific gravities and they average a little lower than those here given. The 

 figures from Pirsson, however, probably express more closely the relation of the 

 petrologic type and the more compact states of rocks to their density. They 

 are, therefore, thought to be better representative of the lithosphere.] 



These figures show that notable departures may occur from 

 the mean density of the outer crust and suggest furthermore that 

 2.67, the mean density used by Hayford, is lower than the actual 

 mean. A more thorough analysis of the subject is therefore needed. 



The abyssal igneous rocks and metamorphic rocks are almost 

 without pore space. The sedimentary rocks, on the other hand, 

 possess abundant pore space in their unconsolidated states, very 

 little in their compact states. The latter is the usual mode of 

 occurrence in the older geological formations. The density is 

 therefore a function of both mineral composition and porosity. 

 The chemical compositions of the several rock types and also of 

 the average sediment and the average igneous rock are well known. 

 The mineral compositions are less well knowh but may be computed 

 with a fair degree of accuracy; the densities, on the contrary, are 

 least commonly reported and the mean densities of the rock types 

 cannot in consequence be closely determined by averaging numerous 

 determinations, as is done for the chemical compositions. It seems 

 desirable, therefore, to compute the densities of the rock types 

 from the chemical and mineral compositions, combining this with 

 the densities of the individual minerals, making a separate correc- 



