214 



JOSEPH BARRELL 



as building stones, but this affects appreciably the density of only 

 a superficial layer and chiefly of the youngest deposits. 



The ratio of shale, sandstone, and limestone in the average 

 sediment in percentage is, according to Mead,' shale 80, sandstone 

 II, limestone 9. The ratio of average porosities in percentage is, 

 according to Fuller,^ crystalline rocks 0.2, shales 4, sandstones 15, 

 limestones 5. The figure given by Fuller for shale rests upon a 

 single determination of 7 . 8 per cent by Delesse, and is averaged 

 in by Fuller with slate. Eight per cent porosity will here be 

 assumed as probably a better estimate. This gives the porosity 

 of the average sedimentary rock as 8 . 5 per cent. The pore space 

 may be taken, following Fuller's estimate, as half filled with water. 



From these data the specific gravities are computed to be as 

 follows : 



TABLE VIII 



Specific Gravities Computed from Mineral Compositions 



*The same figure as used by Chamberlin and Salisbury, Geology, I (1904), 538; also by Pirsson, 

 Rocks and Rock Minerals; also by G. H. Darwin as the density of the outer crust. 



Where Cenozoic deposits occur in thickness, they are consider- 

 ably compacted except at the surface, but still the mean specific 

 gravity, owing to the abnormal pore space and deficiency in lime- 

 stones, is doubtless less than 2.50; 2.45 may be taken. It is 

 probable, on the other hand, that the Paleozoic rocks on the whole 

 have somewhat less pore space than this average, especially as the 

 porosity figure for sandstone rests mainly upon determinations for 

 brownstone, a rather porous type; 2.55 may then be taken as the 

 average for Mesozoic and Paleozoic formations. The pre-Cambrian 



'"Redistribution of the Elements in the Formation of Sedimentary Rocks," 

 Jour. GeoL, XV (1907), 238-56. 

 » Loc. cit. 



