34 The American Geologist. 
Jul.v, 1905 
have produced the broad areas of the great plateaus. It is 
difficult of demonstration, but the impression is very strong 
that these plains are not peneplains cut at sea level, but 
that they were produced at their present altitude by some 
process more or less analogous to the preceding. 
Whatever the factors affecting the region as a whole, 
there seems no manner of doubt that the conoplain of the 
Ortiz has been produced in some such manner. There is 
no evidence whatever of the presence of any large lake or 
sea that could have afforded even a temporary baselevel for 
the cutting. Nor is there any evidence that the country 
has been reduced to a lower level than it has at present, 
since the Miocene. We are forced to the conclusion that 
the sloping plains surrounding mountain masses were cut 
at their present altitude, and that diminishing volume was 
the essential factor in the cutting. 
GENETIC AND STRUCTURAL RELATIONS OF THE IGNEOUS 
ROCKS OF THE LOWER NEPONSET VALLEY, 
MASSACHUSETTS.* 
By W. O. CROSBY, Boston, Mass. 
INTRODUCTION. 
The Lower Xeponset valley, or more specifically, that 
part of the valley of the Xeponset river within the limits 
of the Boston basin, properly embraces all that part of the 
Boston basin between the Blue hills, a denuded anticlinal 
axis dividing the Boston basin from the parallel and over- 
lapping trough of Carboniferous sediments known as the 
Norfolk basin, and the broad band of conglomerate extend- 
ing westward from Savin bill on Dorchester bay through 
Dorchester. Roxbury, West Roxbury, Brookline and New- 
ton into Wellesley and Needham. This great belt of con- 
* This paper is an advance presentation, in outline, of a portion of 
Part iv of the author's detailed and systematic study of the Geology 
of the Boston Basin in course of publication in the series of Occasional 
Papers of the Boston Society of Natural History. For the petrographic 
distinctions in this field the author is indebted to Dr. Florence Bascom, 
whose preliminary observations on the volcanics only have been pub- 
lished (Bull. Geol. Soc. America, vol. 11, 115-126), and whose more com- 
plete and elaborate work on both the volcanics and plutonics awaits 
publication in connection with the forthcoming Part iv of the Boston 
Basin scries. 
