MEMBERS OF THE PYROCRYSTALLINE CLASS. 
69 
of the mass. It approaches gneiss in its structure, but differs 
from it in wanting the arrangement of the mica planes. This 
example, however, proves that a sheeted structure is not due 
to the action of water, and was never arranged into beds like 
the sedimentary rocks. 
The granites of the New England states lie in ranges, 
inclined upon the eastern slope of the Hoosick Mountain range. 
At the south-western corner of Vermont a field of granite forms 
a portion of the western side of the Hoosick mountain, upon 
which repose the lower members of the Taconic system. But 
most of the New England granite lies adjacent to the Atlantic 
coast. It is divisible into those granites which have been 
erupted from comparatively narrow fissures in gneiss, mica 
slate, and an older granite, or those granites which have 
erupted from fissures, but which seem to have overflowed wider 
areas, and whose structure is more or less sheeted, like the 
granite of Hallowell in Maine; and lastly, those granites 
which are still more widely spread, and more ancient than the 
preceding. It is impossible, however, to draw lines of distinc¬ 
tion between the last two kinds of granite, except when the 
last is traversed by the preceding granites in veins. 
§ 57. Granites of the Appalachians. The granites and 
sienites of the southern Highlands upon the Hudson river 
resemble those of New England. They pass southerly into 
Pennsylvania, some of which contain zircons and octahedral 
iron. In the county of Philadelphia the gneiss and mica slate 
is traversed by coarse veins of granite, the feldspar of which 
decomposes into kaolin. So near Manayunk, the mica slate 
which abounds in garnets is intersected by numerous veins of 
granite, in which feldspar predominates. Chester and Lancas¬ 
ter counties furnish numerous localities of granite. 
In Virginia, granite and sienite are not common rocks 
Those which occur form a part of the Blue ridge. Both occur 
in Halifax and Campbell counties on Staunton river and Whip¬ 
ping creek; also on James river four miles above Greenway. 
This is a formation of sienite, and is well adapted for works 
