172 WILLIAM J. MILLER 
In all 11 diabase dikes were found, these being well scattered 
over the quadrangle. Most of them cut across the foliation of the 
country rock at high angles, thus differing from the gabbros, and 
they probably have been forced up along joint planes. In 9 of 
the 11 occurrences the dikes strike northeast and southwest which 
is quite the rule for such dikes in the eastern Adirondacks. So far 
as can be determined, these dikes all come up vertically through 
the country rock. 
MEGASCOPIC AND MICROSCOPIC FEATURES 
The diabase is a very dark bluish-gray to almost black rock 
which, in all exposures, is hard and fresh except for the immediate 
surface which is often weathered to reddish-brown. 
The granularity and texture vary from glassy to very fine 
grained to medium grained diabasic, the finer grained rock being 
wholly confined to the borders and the diabasic texture nearly 
always being just visible to the naked eye in the typical medium 
grained rock. Except for the above named differences the diabase 
shows no facies whatever visible to the naked eye, and this again 
is in marked contrast with the gabbros. 
The diabase is wholly devoid of any metamorphism and inclu- 
sions of country rock are never found. The only minerals recog- 
nizable by the naked eye are the tiny feldspar laths and an occa- 
sional pyrite speck. 
The whole range in mineralogical composition is brought out 
in the following table. The figures refer to percentages by volume 
and are meant to be close approximations only. 
The remarkable similarity in composition and the small number 
of minerals represented stand out in marked contrast against the 
gabbro and its facies. Nos. 1, 2, and 3 are typical holocrystalline 
diabases from widely separated dikes. Nos. 4 and 5 represent 
finer grained or border phases and have more or less glassy ground- 
mass. No. 5 presents a striking appearance under the microscope 
because the feldspar crystals which are incipient and almost indeter- 
minate tend toward sheaf-like bundles. 
No. 1 of Table III, which may be regarded as typical of all the 
diabases, is from the large dike (above described) at the base of 
Heath Mountain. The fine to medium grained rock shows an 
