44 
gite is apparently the chief constituent. Under the microscope, 
however, the section is found to present, aside from the apatite 
a fresh and unaltered 
needles, not a single original mineral ir 
state ; but the entire mass of the rock is filled with small amor- 
phous grains and dust-like opaque particles of a black color, in- 
terspersed throughout a dirty grayish groundmass, with only 
here and there a small fragment of an augite crystal or colorless 
portion of feldspar, in which in a few instances twinning striæ 
were still apparent. Scattering grains of epidote and shreds of 
a brownish mineral, evidently hornblende, complete the list 
of determinable minerals. The black opacite and the gray 
amorphous material are evidently derived from the decomposed 
augite ; it being not infrequent to find the border of a crystal 
irregularly outlined by the larger grains, while interiorly is the 
gray material and other black grains. The form and size 
of the black grains is such as to render their identification by 
the microscope alone impossible. The pulverized rock is, how- 
ever, strongly magnetic; and after long digestion with hydro- 
chloric acid the solution gives a faint reaction for titanium. 
The augitic alteration would in this case seem to be similar 
to that of the hornblende in certain New Hampshire diorites 
described by Hawes.! That the altered mineral in this case is 
augite and not hornblende is proved by a few unchanged parti- 
cles still remaining and the form of the outlines still preserved. 
Viridite, so abundant in the other rocks described, is here en- 
tirely lacking. 
DETAILED STRUCTURE OF NANTASKET. 
The main purpose of this section is to set forth as fully as 
may seem desirable the facts upon which the generalizations of 
'Geol. of New Hampshire, Vol. III., part IV., pages 43 and 66. 
