No. 18. — On the Classification of Rocks* By M. E. WADSWORTH. 
Tax results sketched in this paper, which will be given in greater 
detail elsewhere, were derived from the study of collections made in 
California, Lower California, Arizona, Nevada, Oregon, the territory 
covered by the Exploration of the Fortieth Parallel, the Lake Superior 
Region, New England, Costa Rica, and in Europe. The consecutive 
western area represented by these collections includes some three hun- 
dred thousand square miles, extending over one thousand miles in each 
direction, As this comprises one of the most extensive volcanic regions 
on the globe, which also contains old crystalline and sedimentary rocks, 
it affords a wide field for generalization. 
Knowing the tendency of the human mind to fall into error, I can 
but feel that in avoiding the mistakes of others I may have committed 
greater ones myself. However this may be, my duty is none the less 
plain to give the facts as I understand them, let the result be what it 
will. 
This work has been done at the private expense of Professor J. D. 
Whitney, in continuation of his work upon the State Geological Survey 
of California, and will be published in full in the Memoirs of the Museum 
of Comparative Zodlogy at Harvard University. If this paper has any 
value, the credit belongs to Professor Whitney, as without his generous 
liberality the work could not have been done; if valueless, the blame 
rests upon me, as I have had entire freedom in this work, and no one 
but myself is responsible for a single sentence contained herein. 
My thanks are due to Mr. Clarence King, for his great courtesy in 
allowing me to examine, for some weeks, the rocks and. slides belonging 
to his former Survey, which had been studied and reported upon by 
Professor Zirkel, and for his placing at my disposal his microscope 
during the examination; to the officers of the American Museum of 
Natural History in New York, in whose building the collection was 
deposited, for their efforts to facilitate my work ; also to Mr. George W. 
Hawes of Yale College, for the privilege of examining his New Hamp- 
shire collection. 
* Abstract of a thesis presented for the degree of Doctor of Philosophy in Harvard 
University, March 25, 1879; also given before the Boston Society of Natural History, 
May 7, 1879. The references and the numbers of the sections, which are fully given in 
the original, are omitted here. 
