28 EDWIN C. ECKEL 



at present inclined to believe that these unexpected results are 

 susceptible of a rational, though decidedly novel, explanation. 

 In case this belief is supported by the results of investigations 

 now in progress, this explanation will be presented in a later 

 paper. 



Compariso7i of '^average slate'' and '^average Paleozoic shale'' 

 analyses. — As thirty-five of the thirty-six slate analyses are of 

 Paleozoic material, the "average slate" will obviously be com- 

 parable most directly with the composite analysis of the fifty-one 

 Paleozoic shales. The necessity for restricting the comparison 

 in this manner is accentuated by the fact, above intimated, that 

 the Paleozoic and the later shales are not themselves directly 

 comparable. 



The average slate contains 60.64 P^r cent, of silica, as against 

 60. 1 5 in the Paleozoic shale. Alumina and titanic oxide together 

 amount to 18.05 P^"" cent, in the slate, and to 17.21 per cent, in 

 the shale, the titanic oxide being practically the same in both. 



While the proportions of ferrous and ferric iron oxide in the 

 slate and shale are reversed, the total amount of iron oxides is 

 very close, being 6.87 in the slate and 6.94 in the shale. The 

 lime and magnesia show the first interesting difference, and this 

 is but slight. While their ratios are closely alike in the two 



rocks ( p°- =1.688 in the slate, and 1.646 in the shale), the 



total lime and magnesia in the slate is 4.14 per cent., and in the 



shale 3.73 per cent. A somewhat similar case occurs with the 



K O 

 alkalies. Here '^ „ =3.101 in the slate, and 3.564 in the 



shale; while the total alkalies amount to 4.74 and 4.61 per cent, 

 respectively. 



The variations in carbon dioxide, water of combination, and 

 moisture are so slight that they would not appreciably affect the 

 percentages of the other constituents, with the exception of silica 

 and alumina. 



Stimmary of results. — The results of the comparison are mainly 

 negative, but they are of value even in that way. 



The average slate is practically identical in composition with 



