THE GEOLOGICAL HISTORY OF NEW YORK STATE 33 
very similar to graphite, occurs in the Carbonic strata of Pennsyl- 
vania, and is derived from plants through the process of carboni- 
zation. Graphitic anthracite of like origin occurs in a smaller way in 
Rhode Island. Hence it seems likely that the graphite of the Gren- 
ville represents the remains of plants, probably of the seaweed type 
since there is much evidence against the view that any of the higher 
land plants existed at that very early time. This by no means proves 
the absence of animals from the Grenville ocean, because animals 
with only soft parts would have left no record, while calcareous or 
silicious shells would doubtless have been recrystallized by the 
severe processes of metamorphism to which these rocks have been 
subjected. 
EARLY PRECAMBRIC IGNEOUS ACTIVITY 
After the accumulation of the Grenville sediments, igneous activity, 
took place on a large scale, when great masses of molten rock were 
pushed or intruded into the sediments from below. Several differ- 
ent times of igneous activity have been definitely recognized and the 
general effect of the great invasions of molten rocks was to break 
the Grenville up into patches. In many cases considerable masses 
of Grenville were pushed aside or displaced by the molten masses 
while, to a greater or less extent, there may have been an actual 
melting in or assimilation of Grenville rocks by the molten intru- 
sions. As we have already learned, igneous rocks are those which 
have cooled from a molten condition, and of these there are two 
important types, called respectively, plutonic and volcanic. Both 
. of these types of igneous rocks are found in the Adirondacks, but 
the plutonics are by far the more prominently developed. 
So far as we know, the first great intrusion of molten rock in the 
Adirondacks is represented by the present large area of so-called 
anorthosite in Essex and Franklin counties. This is a very 
coarse-grained, plutonic rock of bluish gray color when fresh and 
consists chiefly of a feldspar (labradorite). The intrusion was 
practically confined to a single area comprising about 1200 square 
miles. That this rock is younger than the Grenville is demonstrated 
by the fact that tongues of the anorthosite have been observed cut- 
ting through the Grenville (see figure 12). This intrusion differs 
from the later intrusions in that it was practically a single great mass 
which broke its way through the Grenville in but one place in the 
whole Adirondack region. In a few cases small patches of Gren- 
ville were caught in the molten flood and may now be seen within 
the anorthosite mass. For the most part, however, the molten rocks 
