AMERICAN SHALES AND ROOFING SLATES 



27 



Average chemical compositio7i of American shales. — In Bulletin 

 168, U. S. Geological Survey, on pp. 16, 17, Dr. Clarke has 

 quoted a series of composite analyses of American sedimentary 

 rocks. The material was selected and the samples were pre- 

 pared by Mr. G. K. Gilbert, assisted by Mr. G. W. Stose, and 

 the analyses were made by Dr. H. N. Stokes in the chemical 

 laboratory of the Survey. 



These composite analyses, so far as they relate to shales, are 

 reprinted here as Table II. The determinations of a number of 

 minor constituents are omitted. In this series each individual 

 shale was taken in amount roughly proportional to the mass of 

 the formation which it represented. The samples were then 

 mixed into one uniform sample from which, by a single analysis, 

 an average composition was determined. 



In column i is given the result of an analysis of twenty-seven 

 Mesozoic and Cenozoic shales ; and in column 2 that of fifty- 

 one Paleozoic shales. Column 3 gives the average of these two 

 determinations, giving them, respectively, weights as 3 to 5. 

 The values in this column are, therefore, an approach to the 

 "average shale " composition, 



TABLE II. 

 COMPOSITE ANALYSES OF AMERICAN SHALES. 



Silica (SiOg) 



Alumina (AI2O3) 



Titanic oxide (TiOg). • ■ 

 Ferrous oxide (FeO) . . . . 

 Ferric oxide (FcgOg) . . . 



Lime (CaO) 



Magnesia (MgO) 



Potash (KgO) 



Soda(NaaO) 



Carbon dioxide 



Water of combination . . 

 Moisture, below iio° C. 



55-43 



13.84 



0.46 



1.74 

 4.00 

 5.96 

 2.67 

 2.67 

 1.80 

 4.62 



3-45 

 2. II 



60.15 

 16.45 

 0.76 

 2.90 

 4.04 

 1. 41 

 2.32 

 3.60 

 1 .01 

 1 .46 

 3.82 

 o.8q 



58.38 

 15.47 

 0.65 

 2.46 

 4-03 

 3. 12 



2.45 

 3.25 

 I-3I 

 2.64 

 3-68 

 1-34 



It may be noted in passing that some of the differences in 

 composition between the Paleozoic and the later shales were, 

 either in degree or in kind, contrary to what might have been 

 expected, from a purely theoretical standpoint. The writer is 



