PYROCRYSTALLINE ROCKS. 
59 
sometimes in regular hexahedral tables, is one of the most 
common minerals of pyrocrystalline limestone. When it is 
considered that graphite is a furnace product, we can scarcely 
entertain a doubt respecting the agent which was instrumental 
in its origin. This mineral, however, is absent in those rocks 
which are usually regarded as the metamorphic limestones, as the 
marbles of Berkshire, Mass., and which are prolonged to Canada 
on the north, and to Georgia on the south. The green crys¬ 
tallized mica, the brown zircons, sphene, corundum, sulphuret 
of iron in fine crystals, prismatic feldspar, crystals of quartz in 
dodecahedrons, carbonate of iron, yellow and brown tourma¬ 
lins, are among the simple minerals of this rock. 
The simple minerals which we find commonly associated 
with certain rocks may be regarded as having originated under 
three conditions: the first, those which belong to the granites 
and pyrocrystalline limestone; these have been developed 
through the agency of high temperature. The second, those 
which belong to the mica and talcose slates; these have 
required for their production only a moderate amount of heat. 
The kyanites, garnet, staurotide, andalusite, belong to this 
series. Third, those which are developed in thin seams (not 
veins) and cavities of rocks, mainly through the instrumentality 
of water. As examples, stilbite, heulandite, thompsonite, cha- 
basic, &c», may be cited. 
The rocks admit of grouping to a certain extent according 
to the period during which they were formed, or according to 
the minerals which enter into their composition. The feld- 
spathic rocks belong to different periods—the most crystalline 
to the earliest, the amorjDhous and subcrystalline to later 
periods. To the former belong the granites, and to the latter 
the greenstones. The schistose rocks, gneiss, mica and talcose 
slates, together with hornblende, are closely related, and were 
evidently formed under a diminished temperature. It was 
temperature, and not water, which arranged their laminee into 
parallel layers, a result which is sometimes imitated in furnace 
operations. We find no sediments beneath, or intercalated 
between them. 
