Relations ot the Igneous Rocks — Crosby. 7o, 
perhaps, to torsional stresses accompanying the shearing. 
The felsite dikes range in size up to a hundred feet or 
more ; and they can be traced in some instances for a good 
fraction of a mile; but the correlation of individual outcrops 
is often difficult because of marked irregularities of form 
and trend. As described by Dr. Bascom, the felsite of these 
dikes is mainly either densely compact or, more commonly, 
inconspicuously porphyritic. In general, and as might nat- 
urally be expected, the larger dikes have porphyritic centers 
and nonporphyritic or compact borders ; while the smaller 
dikes are often nonporphyritic throughout. With few ex- 
ceptions, the peripheral portions of the dikes exhibit more 
or less distinct, and often very marked, fluidal structure 
parallel with the walls ; and the smaller dikes may be char- 
acterized by the fluxion lamination through their entire 
thickness. The large dikes, also, are usually dark red or 
purple in the middle portion and greenish gray along the 
borders ; while the small dikes are commonly gray across the 
entire section. It appears probable that the normal orig- 
inal color of the felsite was gray, that it was subsequently 
reddened by oxidation and later bleached by deoxidation 
and leaching along the borders. The greenish color of the 
periphery is, however, according to Dr. Bascom's observa- 
tions, to be connected, in most cases at least, with a more 
or less marked epidotization, often followed by hydration 
and the development of pinite ; and not infrequently a bor- 
der of nearly pure, soft, green pinite has resulted. 
PRE-CARBONIFEROUS EFFUSIVES. 
Effusive Felsite or Normal Aporehyolite — The acid 
effusions of the vents described in the preceding sections, 
and, doubtless, of other vents still entirely concealed by the 
effusives, probably constitute for the Neponset valley, a 
more or less continuous sheet of lava chronologically and 
stratigraphically intermediate between the denuded surface 
of the batholite and the Carboniferous sediments and dis- 
tinctly unconformable in its relations to both. The petro- 
graphic and chemical characters of the effusive felsites have 
been fully described by Dr. Bascom. The original textural 
variations are most notable, including compact, fluidal, 
spherulitic, and clastic forms. Although the surface ex- 
