82 The American Geologist. 
August, 1»05 
agglomerate, the product, probably, of explosive submarine 
eruptions, are prominently developed. The contempora- 
neous relation of the effusive andesite and the conglomerate 
is especially clear for these fragmental varieties ; but hardly 
less so for the regularly interbedded flows in the Central 
avenue and other parts of the field. The transverse sec- 
tions of individual flows are, in some instances, very in- 
structive, showing a normal gradation upward from densely 
aphanitic to amygdaloidal and scoriaceous forms of lava. 
DIABASE DIKES. 
As previously intimated, the diabase dikes of this area, 
as of the Boston basin generally, are referable to two dis- 
tinct series — distinct in age, trend and lithologic character. 
We may properly emphasize the chronologic distinction, as 
of greatest geologic significance, by designating these two 
series, provisionally, the Carboniferous and the Triassic. 
Evidently, the diabase dikes are not related in origin or 
composition to any of the other igneous rocks of the dis- 
trict; and in size, regularity and continuity the two systems 
are essentially similar and normal. 
Carboniferous Diabase Dikes - The normal trend of 
the numerous dikes of this series is approximately east- 
west; and they rarely vary more than thirty degrees from 
the normal. Although commonly approximately vertical, 
they are more likely than the Triassic dikes to exhibit a dis- 
tinct hade, especially in the sedimentary terranes, the mani- 
fest tendency being to conform with the strike joints of the 
enclosing formation. In other words, these are longitudin- 
al dikes, traversing a series of unsymmetric folds, and sym- 
pathizing in attitude with the tension planes of the flexures, 
having been developed during a period of folding and strike 
or thrust faulting. In the dikes of this series transverse 
columnar jointing is rarely distinctly developed. Litho- 
logically they are rather fine-grained greenstones, the orig- 
inal or normal constituents having suffered extensive chlo- 
ritization and epiclotization, in consequence of which the 
diabase is somewhat immune to kaolinization and to be 
reckoned among the more resistant rocks of the region. 
Triassic Diabase Dikes — The rather infrequent dikes 
of this series adhere very closely to a north-south trend and 
