15 



" Classification of Rocks " is the exponent, Dr. Wadsworth lias 

 found it necessary to enter upon the discussion of other more 

 general problems, so intimately connected with the subject in 

 hand that they could not well be left unexamined. Among 

 these problems were, 1st, the nature and origin of the older 

 crystalline rocks ; 2d, the structure and condition of the earth's 

 interior ; 3d, the nature and origin of meteorites ; and 4th, the 

 nature and composition of the rocks more basic than the basalts. 

 The investigation of the first mentioned of these topics led to 

 the publication of " Notes on the Iron and Copper Districts of 

 Lake Superior," as well as of numerous smaller papers, and 

 finally to a work of larger scope — already mentioned — the 

 joint production of the Sturgis-Hooper Professor and his As- 

 sistant, and entitled, " The Azoic System, and its Proposed 

 Subdivisions." The results of the examination of the second 

 subject mentioned above were published in a communication 

 which appeared in the American Naturalist ; while the ques- 

 tions coming under the head of the third and fourth topics 

 enumerated above were discussed in papers already noticed 

 in previous Reports, and in others indicated in the list here 

 appended. 



Dr. Wadsworth's work on the Cordilleran collections began 

 w T ith the basaltic, andesitic, trachytic, and rhyolitic rocks ; but 

 it has seemed best to refrain from publishing these results until 

 the more basic forms could be studied. Owing to the limited 

 amount of material of the last-mentioned kind in the Museum 

 collections, it became necessary to study the work of others, and 

 unite the results thus obtained with those reached by the aid 

 of our own collections, and the result is, that in systematically 

 arranging the whole mass of material for publication, it has been 

 found best to divide the proposed volume into three portions, as 

 follows : — 



1st. A general discussion of the principles of the work, and a 

 study of the groups more basic than the basalts. 

 2d. The basalts and andesites. 

 3d. The trachytes, rhyolites, and jaspilites. 



The chief portion of the second division of the work has been 

 lying in manuscript since 1879, and the first part is, as already 

 mentioned, nearly ready for delivery. 



