16 ANALYSES OF ROCKS, U. S. GEOLOGICAL SURVEY. [bull. 168. 
discussed, and the subjoined percentages, which are probably nothing 
more than rough approximations to the truth, were obtained: 
Quartz 12. 
Feldspars 60. 
Pyroxenes and amphiboles 18. 
Micas 4.0 
94.0 
The less frequent minerals make up the remaining 6 per cent. The 
computation, although it is by no means conclusive, is perhaps a little 
more satisfactory than any similar estimate which has so far been 
made. 
For computing the average composition of the sedimentary rocks 
the existing analyses of individual samples are inadequate. They are 
too few and too incomplete to yield any conclusions of value. 
Attempts have been made to partly use the data, as, for example, by 
Joly; 1 and it seems probable, therefore, that better material will not 
be without interest or scientific value. 
Some five years ago, at the request of Mr. G. K. Gilbert, a series of 
composite analyses of sedimentary rocks was made in this laboratory. 
Man} r samples were mixed into one uniform sample, from which, by a 
single analysis, an average composition was determined. The mate- 
rial was selected and the samples were prepared by Mr. Gilbert, assisted 
by Mr. G. W. Stose, and the analyses were made by Dr. H. N. Stokes. 
The data obtained may be tabulated as follows: 
A. Composite analysis of 27 Mesozoic and Cenozoic shales. Each individual shale 
was taken in amount roughly proportional to the mass of the formation which 
it represented. 
B. Composite analysis of 51 Paleozoic shales, weighted as in the former case. 
C. General average of A and B, giving them, respectively, weights as 3 to 5. This 
average represents 78 rocks. 
D. Composite analysis of 253 sandstones, about one gramme of each being taken in 
preparing the average sample. 
E. Composite analysis of 371 sandstones used for building purposes. Equal weights 
taken. 
F. Composite analysis of 345 limestones, equal weights being taken. 
(t. Composite analysis of 498 limestones used for building purposes, equal weights 
taken. 
An estimate of the geological age of the earth: Sci. Trans. Royal Dublin Soc, vol. 7, p. 23, 1899. 
