36 NEW YORK STATE MUSEUM 
It should be stated that this period of igneous activity is by no 
means confined to the Adirondacks. Similar intrusions are known 
in the Highlands-of-the-Hudson and in Canada. The covering of 
Paleozoic strata in southwestern New York prevents direct observa- 
tion, but all things considered, it is more than likely that much or 
all of that part of the State, stripped of the Paleozoics, would also 
show Grenville cut up by granite and syenite. 
FOLDING OF THE ROCKS AND UPLIFT OF THE ADIRONDACKS 
At some time after these great periods of igneous activity and cool- 
ing of the rocks, the whole Adirondack region was subjected to an 
enormous pressure as a result of which the rocks were highly folded 
and compressed. The Grenville strata now seldom lie horizontally 
but are tilted at all sorts of angles, and even the igneous rocks thus 
far described, show unmistakable evidence of having been greatly 
compressed because the minerals are flattened out or arranged in 
parallel fashion, often exhibiting a crude, banded structure. Rocks 
which have thus been changed are known as gneisses and may be 
either igneous or sedimentary. 
“Still later than the granites and syenites, there occurred intrusions 
of gabbros in the form of dikes or fissures filled with igneous rock. 
This gabbro is a very dark gray, rather coarse-grained, plutonic 
rock, and it has been found in numerous small masses throughout 
the Adirondacks, but especially in the eastern portion. Its age, 
younger than the granite-syenite series, is demonstrated by the fact 
that it often breaks through those rocks, and also because it is not 
nearly so much metamorphosed. 
The Grenville sediments were completely crystallized as a result 
of this metamorphism so that all beds of limestone were converted 
into marble, sandstone into quartzite, and the shales into gneisses of 
varying character. Thus the Grenville strata have been very greatly 
altered from their original condition, which explains why they do 
not look like the more typical and familiar sediments of later age. 
The manner in which the Grenville strata, especially the limestones, 
were crumbled and folded shows that the rocks must have been in a 
more or less plastic condition when the pressure was exerted (see 
plate 13). Heat and moisture, no doubt, aided in this process 
which we call metamorphism. Such a process can take place only 
at thousands of feet below the surface of the earth where the rocks, 
under the enormous weight of overlying material, would act like 
