THE AVERAGE IGNEOUS ROCK 779 



The most important conclusion is that the average analyses 

 of Clarke, Harker, and Washington are closely checked by this 

 method, and that of the three analyses Clarke's average is most 

 closely approximated. The nature of the data obviously does 

 not permit of drawing any definite conclusions as to the cause of 

 the slight differences between Clarke's averages and the averages 

 here obtained. 



The greatest difference occurs for MgO, the combined granite 

 basalt showing 30 . 5 per cent less MgO than Clarke's average. 

 This difference is so much greater than for any of the other oxides 

 that it may be of significance. It is possible that igneous rocks 

 collected largely for petrographic purposes would include the 

 unusual and interesting alkaline and ultra-basic types, the syenites, 

 peridotites, etc., in an amount out of proportion to their abundance 

 as compared with the more common granite and basalt types. 

 The alkalies, lime, silica, and alumina would not be materially 

 affected by this, as the alkaline rocks would offset the basic varieties, 

 but the range of MgO content being much greater (varying from i 

 per cent or less in the alkaline rocks to 30-40 per cent in the peri- 

 dotites) , this oxide would tend to be increased by the inclusion of an 

 excess of the less common rocks. 



Ratio of shale, sandstone, and limestone. — ^The foregoing solu- 

 tion, while yielding rather definitely the ratio of granite and basalt 

 best explaining the composition of the sediments, simultaneously 

 yields the ratio of shale, sandstone, and limestone resulting from 

 the redistribution of this hypothetical average igneous rock. In 

 Fig. 3 the lines for the several oxides were obtained by drawing 

 lines parallel and between the granite and basalt lines on Fig. i, 

 at distances representing the 65-35 ratio. These lines in Fig. 3 

 intersect within a small area and a point at the center of this area 

 expresses a ratio of 88 parts shale, 9 parts sandstone, and 3 parts 

 limestone. Since water and carbon dioxide were excluded from 

 the analyses of the sediments in obtaining these ratios, it is neces- 

 sary to recalculate to include these constituents, which gives 87 

 parts shale, 8 parts sandstone, and 5 parts limestone. This ratio 

 differs from the ratio of 80 : 1 1 : 9 previously obtained by the writer,^ 



I W. J. Mead, op. cit. 



