24 CLARKE— STATISTICAL METHOD 



[April 18 



assume this matter to be all sedimentary, which it certainly is not, 

 and add to it any probable allowance for the sediments at the 

 bottom of the sea, we shall still fall far short of the half-mile shell, 

 wdiich, on chemical evidence, is a maximum. In the following cal- 

 culation this maximimi will be taken for granted. 



The relative proportions of the different sedimentary rocks 

 within the half-mile shell can only be estimated approximately. 

 Such an estimate is best made by studying the average igneous 

 rock, and determining in what way it can break down. A statis- 

 tical examination of about 700 igneous rocks, which had been de- 

 scribed petrographically, leads to the following rough estimate of 

 their mean mineralogical composition : 



Quartz 1 2.0 



Feldspars 59.5 



Hornblende and pyroxene 16.8 



Mica 3.8 



Accessory minerals 7.9 



lOO.O 



The average limestone contains 76 per cent, of calcium carbonate, 

 and the composite analyses of shales and sandstones correspond to 

 the subjoined percentages of component minerals : 



Shale. Sandstone. 



Quartz^ 22.Z 66.8 



Feldspars 30.0 1 1 .5 



Clay- 25.0 (>.(i 



Limonite 5.6 1.8 



Carbonates 5.7 ii.i 



Other minerals 11.4 2..2 



100. o 100. o 



If now we assume that all of the igneous quartz, 12 per cent., has 

 become sandstone, it will yield 18 per cent, of that rock, which is 

 evidently a maximum. Some quartz has remained in the shales. 

 One hundred parts of the average igneous rock will form, on de- 

 composition, less than 18 parts of sandstone. 



The igneous rocks contain, as shown in the last analysis cited, 



^ The total percentage of free silica. 



^ Probably sericite in part. In that case the feldspar figure becomes lower. 



