LITHOLOGY. 165 
slates, and on fresh fractures it is red. This color results from the sepa- 
ration of iron oxide. The rock is porphyritic; but many crystals have 
rotted away, and the stone is now full of cavities containing calcite. 
Mica Diorite. A diorite in which biotite replaces hornblende occurs 
at Stewartstown. This is not a porphyritic rock, and, in addition to its 
triclinic feldspar, which is probably anorthite, and its biotite, it contains 
much calcite, and also some magnetite, pyrite, apatite, and chlorite. Some 
such calcareous rocks were called hemithrene by Brongniart, but the 
term is now obsolete; and the rocks so called are referred to diabase 
and diorite, to which their nature and composition most closely relate 
them. This specimen of diorite is rather different from all others col- 
lected in the state, and whether it is an original product, or a result of 
decomposition, is questionable. 
GaBsBrRo. 
This rock in its mineral constituents is closely related to diabase, from 
which our varieties are distinguished not only by the circumstance that 
the pyroxene is of the foliated kind which is called diallage, but also by 
their coarse granular structure, which in its details is much more like 
that of granite than that of the diabase that has been described. 
Gabbro is found in immense masses in Waterville, and in the vicinity 
of Mt. Washington. The relationships of its masses to the surrounding 
strata are not so easily determined as are those of the little dykes of dia- 
base and diorite, the walls of which are usually plainly seen; but at some 
points the rock possesses all the structure of an eruptive mass, and when 
in other places this is not found, the evidence furnished by more favor- 
able localities, as well as that furnished by allied rocks in other lands 
where they have been more thoroughly investigated, must at present be 
decisive. : 
Our gabbros are coarse granular mixtures of labradorite, foliated augite 
or diallage, olivine, and magnetic or titanic iron. Apatite and biotite are 
the constant accessories. Hypersthene is sometimes prominent, and 
sphene, chlorite, and pyrites are often present. The first four ingredi- 
ents are macroscopically conspicuous, and the rest are identified in thin 
sections. The prevailing color of the rock is dark gray, but it varies 
