SO-CALLED PORPHVRITIC GNEISS OF NEW HAMPSHIRE 719 
gneiss, some of which are large, being as much as twenty-five 
feet in diameter. Within one of the latter an interesting irregular 
injection of the porphyritic granite is well exposed. One 
feature exceptionally well shown is the fine-graining along the 
margins. At this place, too, dikes of the Concord granite cut 
the porphyritic granite, and the former rock is thus the youngest 
terrane in the region. Since it surrounds the porphyritic granite 
on all sides, this Fitzwilliam occurrence may itself be a large 
floe brought up from below from a much larger mass. 
The Main area—Our observations on the Main area were 
extended only to its southern half. The great thickness of the 
various glacial deposits make it, on the whole, less satisfactory 
for a study of contact relations than the Winnipiseogee area. 
We have aimed in the course of a somewhat hasty examination 
to discuss the facts as regards the schists grouped by the survey 
under the names of the ‘“‘Ferruginous slates,” and the ‘“ Ferru- 
ginous schists.” In both, the rocks consist of two-mica-schists, 
biotite-schist, biotite-gneiss, and muscovite-biotite-gneiss, all of 
which may be garnetiferous. Between them we can trace no 
definite distinction, either of composition or of age, and the area 
on the eastern border, marked ‘‘Lake Winnipiseogee gneiss,” 
encloses stripes of schistose rocks which are identical with those 
of the above-mentioned groups. Here, as often elsewhere, the 
grounds for the subdivision carried out by the survey do not 
appear in the field. 
The great sill of porphyritic granite to the north and north- 
east of Greenfield has one contact well exposed with interrup- 
tions for the distance of amile. It is an intrusive one. The 
granite was erupted into the schists along a plane of foliation, 
and there is the usual development of parallel structure in many 
of its outcrops which accords with that of the walls. The 
intrusion is, on the whole, sheet-like in its form, though in some 
places where the schists are intensely folded, it cuts across their 
structure-planes. In such cases the foliation of the porphyritic 
granite remains parallel to the boundary line. The apophyses 
penetrate the schists irregularly in all directions, generally pay- 
