No. 13. — On the Classification of Rocks* By M. E. Wadsworth. 



The 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 Zoology 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. 



