34 NEW YORK STATE MUSEUM 
either completely pushed aside the Grenville or melted it into itself, 
the former being the more likely. Many of the highest points within 
the Adirondacks, like Mt Marcy, are in this anorthosite area. 
The next clearly recorded event, after the anorthosite intrusion, 
was very widespread igneous activity when the rocks of the granite- 
syenite series, now so well known in the Adirondacks, were forced 
upward into the Grenville sediments. To be precise, there were at 
least two or three periods of intrusions of such rocks, the oldest 
probably being represented by the so-called Laurentian granite of 
the Thousand Islands region. For our purpose it will suffice to re- 
gard these as having all been intruded at about the same time, since 
the sum total of effects is much the same as though there had been 
but a single period of activity. Granite is a plutonic, igneous rock 
which consists essentially of quartz and feldspar (orthoclase), 
v. 
4 xx 
mew Grenville Aone hoaire eae +44,4]Gebbro or 
K series wXx" x vr series + +" diabase dife 
Fic. 12 Generalized section showing the relations of the most common 
Precambric (Adirondack) rocks to each other and how their relative ages 
are determined. 
together with more or less ‘black mica, hornblende, or augite. Syenite 
is the same except that quartz is much less prominent or lacking. 
The fresh rock is of a greenish gray or pinkish gray color, while 
on weathered surfaces the color is usually light brown. Many of 
the highest mountains outside the anorthosite area are of granite or 
syenite. 
The present distribution of these rocks shows that the molten 
masses broke into the Grenville in very irregular fashion, sometimes 
pushing the Grenville aside; sometimes enveloping great or small 
masses within the molten flood; or, in other cases, apparently leav- 
ing large Grenville masses intact. All portions of the Adirondacks 
felt the force of the intrusions and a detailed geologic map of the 
region would show a decided patchwork effect (see figure 13) due 
to the irregular manner in which the Grenville has been cut up by 
the igneous rocks. These igneous rocks are generally easily distin- 
guished from the old sediments because of their homogeneity in 
