15 
‘* Classification of Rocks’”’ is the exponent, Dr. Wadsworth has 
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 
with 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 : — 
Ist. 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. 
