526 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



Professor Joly's strongest argument is that sedimentary rocks 

 are comparatively low in soda, and that if the soda in the ocean 

 be restored to the sedimentary rocks the original composition, 

 namely that of a dioritic magma, would be restored. In this 

 connection it might pertinently be asked if sufficient analyses of 

 sedimentary rocks have been made in a sufficient number of 

 places to warrant computations being made. In granite-areas, 

 where the sedimentaries have been derived from granite, poverty 

 in soda must be expected, and most analyses of sedimentary and 

 metamorphic rocks have been made in granite-areas. Further, 

 potassium by its tendency to form insoluble compounds in the 

 soil, is largely carried to the ocean in suspension and deposited 

 in sedimentary rocks, but soda by its great solubility is retained 

 in solution until an arm of the ocean is evaporated, or at least is 

 only chemically precipitated in the deepest portion of the ocean 

 where there is a tendency for soda minerals to form. Until we 

 can say that we know the total bulk of common salt in sedimen- 

 tary strata and salt beds in the entire crust of the earth, and 

 we understand thoroughly the nature of deep sea deposits, any 

 computations such as Joly has endeavoured to make are in vain. 



Joly's statement that there is an excess of Na over K in 

 igneous rocks, whilst there is in general an excess of K over Na 

 in sedimentary rocks is liable to strong objections. In the first 

 place the great bulk of rock analyses are of volcanic rocks which 

 are generally more soda-rich than their plutonic equivalents; and 

 sodic rocks, containing as they do many rare minerals, have been 

 more frequently analysed than normal rocks, hence an estimate 

 based on averages of rock-analyses has no quantative value, 

 especially so when we remember that some of the largest areas in 

 the world are made up of potash-rich granite and granite por- 

 phyry. A single analysis of a large area of rock of homogeneous 

 composition suffices for all practical purposes. 



Further, as I have already indicated, the supposed excess of 

 potash over soda in sedimentary rocks cannot be said to be 

 established, inasmuch as a correct estimate has first to be obtained 

 of the volume of all the salt beds in the world, and of all the 



