12 ANALYSES OF BOCKS, U. S. GEOLOGICAL SURVEY. [bull. 168. 
official reports, monographs, bulletins, American and foreign journals, 
and so on. The object of this bulletin is to bring together this valu- 
able material, together with such bibliographic and petrographic data 
as seems to be necessary in order to identify the specimens and to facili- 
tate chemical discussion. Analyses of minerals have been included 
only when related to petrographic studies, appearing then in connec- 
tion with the rocks to which they belong. Meteorites, of which 
twentv-seven have been analyzed, are brought into the work on account 
of their petrographic relations; and the groups of clays and soils have 
been admitted because of the bearing of these substances upon the 
study of slates and shales. The actual number of analyses given in. 
the bulletin is as follows: 
Igneous and crystalline rocks 854 
Mineral separations 90 
Meteorites, and separations from them 58 
Sandstones, cherts, and sinters 48 
Carbonate rocks 208 
Slates and shales 48 
Clays, soils, etc 98 
Total 1, 404 
It may be observed that the classification thus indicated has not been 
rigorously followed. In a few instances the study of a sedimentary 
rock has been so related to that of its igneous neighbors that the analyses 
are best tabulated together; but these exceptional cases are few, and 
all are properly noted. The heading u igneous and crystalline rocks" 
has been used in the broadest and most liberal way, and doubtless 
many of the analyses given under it might properly be otherwise 
classified. In such cases of uncertainty, convenience has furnished 
the rule to follow. 
Within each division of the analyses the classification chosen has 
been geographic. The petrographic grouping of the rocks would 
doubtless be best were petrographers agreed upon it; but their dif- 
ferences are many, and the chemist will do well to avoid them. The 
geographic method, moreover, has some advantages of its own; it 
facilitates the study of areas, it simplifies the bibliographic references, 
and it brings together, in great measure, the work of each petrog- 
rapher for whom analyses have been made. Thus, most of Diller's 
work has been in California, most of Cross's in Colorado, and most 
of Iddings's in the Yellowstone National Park, and in each case the 
analyses are massed, and their discussion is practically uniform in 
character. As regards nomenclature, each rock has received the desig- 
nation given it by its describer, and no liberties have been taken. 
This plan may cause some lack of uniformity; but no other procedure 
seemed to be practicable. 
It will be noticed by anyone who uses this bulletin that the analyses 
vary as regards completeness. Among the sedimentary rocks, espe- 
