ADIRONDACK BASIC INTRUSIVES 161 
The syenite and granite break through the Grenville in batho- 
lithic masses of greater or lesser extent. These rocks, which are 
always foliated, vary from basic syenite through quartz syenite 
and granite to granite porphyry, and all are differentiation prod- 
ucts from the same great cooling magma. About two-thirds of 
the area of the quadrangle is occupied by these rocks. Sometimes 
they are so closely involved with the Grenville as to preclude the 
possibility of separate mapping. 
The gabbro, which exists under many facies, is clearly intrusive 
into the Grenville and syenite-granite series. It is generally more or 
less metamorphosed, especially along its borders where amphibolite 
is often developed. It occurs in numerous small masses. 
Pegmatite dikes show a very close association with the gabbro, 
it being quite the rule to find such coarse grained dikes often of 
very respectable size cutting the gabbros and frequently sending 
small tongues into the latter rock. 
Diabase occurs in small dikes, and in contrast with the gabbro, 
it is remarkably uniform in texture and composition. The writer 
has observed the diabase cutting the pegmatites and, in one case, 
the gabbro. . 
The accompanying geologic map gives a fair idea of the rela- 
tions of the various rocks of the region under discussion. 
THE GABBRO AND ITS DERIVATIVES 
MODE OF OCCURRENCE 
The gabbro and its derivatives nearly always occur in the form 
of small stocks or bosses rather than as true dikes, the length 
ranging from 30 or 40 feet to about a mile, and with widths up to 
3 of a mile. The ground plan as represented on the geological 
map is almost invariably elliptical, though sometimes approaching 
the circular. When the contact with the country rock is carefully 
traced out it is commonly found to be sharp and shows smooth or 
flowing outlines against the country rock. These features are 
pretty well shown on the accompanying map. 
In only two or three cases do the gabbro masses approach the 
true dike-like form, and in each of these fine grained tongues were 
found to extend into the surrounding rock. A good example is 
