scure; and quite large grains of iron oxide and pyrite occur. 
The segregation patches already alluded to are found, under the 
‘microscope, to consist largely of deep brown, strongly dichroic 
hornblende, in compact and well-defined crystals; sometimes 
perfectly fresh or again somewhat altered into the inevitable 
viridite. I am not able from the sections prepared of this rock 
to state whether this hornblende is original or a product of para- 
morphie alteration from the augite. 
Dike 31.—This is of medium fineness, with macroscopic pyrite 
and segregations of black hornblende. The color is greenish, 
the rock being blotched with epidote. The augite is almost 
completely altered to viridite, although a few remnants are. still 
recognizable. This rock also contains, as stated, coarser grained 
segregations in which black hornblendes are readily distinguished 
by the unaided eye. In thin sections these are of a deep brown 
color and strongly dichroic. Some of the individuals are so 
compact and well defined in crystalline outline as to indicate 
that they are original constituents of the rock ; while in other 
cases the presence of a hornblende border of varying width about 
an augitic core indicates unmistakably a paramorphic origin. 
The hornblende has, in its turn, undergone in certain cases the 
viriditic alteration. This is the only one of the Nantasket rocks 
in which I have been able to find paramorphic hornblende so 
distinctly characterized as to leave no doubt of its origin, al- 
though this is suggested by sections from dike 29. 
A few small, grayish, wedge-shaped crystals of sphene are 
present. Feldspars, in most part, are considerably altered ; 
but in a few instances are still clear and show the twinning 
strio very plainly. There is much viridite, which obscures 
everything. 
Dike 34.—Differs in no essential particulars from 29 and 31, 
though the section does not show the hornblendic segregations. 
These two dikes are without doubt iden- 
Dikes 40 and 41. 
tical with the preceding, though nothing in the present compo- 
