2IO The America?i Geologist. April, 1900 
American Association for the Advancement of Science, Proper 
Objects of. Pop. Sci. Mo. Aug. 1899. v. 55, pp. 462-472. 
Rock waters of Ohio. Geological Survey of U. S. 19th Annual 
Report. 1899. pt. 4. pp. 633-718. 
Human progress in the nineteenth century. (Address of the Pres. 
of the Am. Asso'n for the Adv. of Sci.) Science, v. 10 N. Ser. pp. 267- 
271. 1899. 
CLASSIFICATION OF IGNEOUS ROCKS ACCORD- 
ING TO COMPOSITION.* 
By J. E. Sfcre, Wasbing^ton, D. C. 
In Studying the peculiarities of the many varieties of rocks 
collected in southwestern Alaska during the season of 1898 a 
scheme of classification was forced upon the writer as being 
the only natural expression of the relations and characteristics 
of these rocks as understood by him. In the course of this 
study, as well as in his previous study of igneous rocks, he 
has naturally kept in mind various conflicting systems of 
classification and the ideas of many petrographers. Of these 
ideas he has accepted some, rejected some, modified some, and 
has also formulated some new, his desire being simply to ex- 
press facts as he saw them. The necessity for writing clearly 
and intelligently about the Alaskan rocks made him draft a 
classification for those collected in 1898. The idea in this was 
simply to elucidate the nature and relations of these Alaskan 
rocks and no idea was entertained of proposing a classification 
which should find general favor. At the same time the writer 
realized that a classification, which should express faithfully 
the relations of so great a variety of rocks as those described 
in this report, must also, if it be a true one, be capable of 
being extended or modified so as to embrace other rocks, and 
bv comparative research he had already settled in his own 
mind that the classification would stand this test. After the 
♦Written in June, iSgq, as a part of a report on "A Reconnaissance 
in Southwestern Alaska," which appears in the Alaska volume of the 
forthcoming Annual Report of the U. S. Geological Survey (Twentieth 
Annual). The portion here presented was afterwards separated from the 
rest of the report, and is published herewith the permission of the di- 
rector. For the detailed description of the rocks involved in this dis- 
cussion the reader is referred to the aliove mentioned report. 
