10 ANALYSES OF ROCKS AND MINERALS, 1880-1908. 
igneous rock has very nearly metasilicate ratios and is close to an 
andesite in composition. Its place in the new classification of rocks 
has been fully discussed by Washington in the memoir already 
mentioned. 
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. At- 
tempts have been made to partly use the data, as, for example, by 
Joly ;* and it seems probable, therefore, that better material will not 
be without interest or scientific value. 
About fourteen years ago, at the request of Mr. G. K. Gilbert, a 
series of composite analyses of sedimentary rocks was made in this 
laboratory. Many samples were mixed into one uniform sample, 
from which, by a single analysis, an average composition was deter- 
mined. The material 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. 
. Composite analysis of 51 Paleozoic shales, weighted as in the former case. 
General average of A and B, giving them, respectively, weights as 3 to 5. This 
average represents 78 rocks. 
. Composite analysis of 253 sandstones, about 1-gram of each being taken in pre- 
paring the average sample. 
: ee mad analysis of 371 sandstones used for building purposes. Equal weights 
taken. — 
. Composite analysis of 345 limestones, equal weights being taken. 
; ve analysis of 498 limestones used for building purposes. Equal weights 
taken. 
Os fo OW 
H20 at 110° 
H20 above 110° 
i 
Carbone 
100. 48 | 100.46 | 100.46 | 100.41 | 99.86 | 100.09 | 100.34 
a An estimate of the geological age of the earth: Sci. Trans. Royal Dublin Soc., vol. 7, 1899, p. 23. 
b Includes organic matter. 
¢ Of organic origin. 
