20 ANALYSES OF BUCKS, U. S. GEOLOGICAL SURVEY. [no. 228. 
rutile the calculation was .simple; but the other figures in the follow- 
ing table are approximative only: 
Apatite 0. 6 
Titanium minerals 1.5 
Quartz 12. 
Feldspars 59. 5 
Biotite '- - 3. 8 
Hornblende and pyroxene 16. 8 
94.2 
The less frequent minerals make up the remaining 5.8 per cent. 
The computation, although it is by no means conclusive, is not without 
some significance. It is interesting to note that the average igneous 
rock has very nearly metasilicate ratios, and is quite 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 anatysesof 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; rt and it seems probable, therefore, that better material will not 
be without interest or scientific value. 
Some nine years ago, at the request of Mr. G. K. Gilbert, a series 
of composite analyses of sedimentary rocks was made in this labora- 
tory. Many samples were mixed into one uniform sample, from which, 
by a single analysis, an average composition was determined. 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. 
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 1 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. 
G. Composite analysis of 498 limestones used for building purposes, equal weights 
taken. 
« An estimate of the geological age of the earth: Sei. Trans. Royal Dublin Soe., vol. 7, 1899, p. 23. 
