Dikes of the Hudson River Highlands. 695 
Orange county, which may prove on further examination to be 
similar. The writer hopes, as opportunity occurs, to add to our 
knowledge of them. 
Beyond Fort Montgomery, except in the case just cited, no dike 
rocks are to be seen until Cro’ Nest Mountain is reached, north of 
West Point. The interval is made up of gneisses, very feldspathic, 
and considered by Dr. Britton to belong to the Upper Schistose 
Series. They have peculiar feldspathic masses in them which must 
be considered in any question of origin or metamorphic action. 
Great cleavage faces of feldspar as large as the hand reflect the 
sunlight from the sides of the cuts. Cro’ Nest and Storm King 
form the northern boundary of the Highlands west of the Hudson. 
They consist of rocks which are described by Dr. Britton! as 
quartz-syenite,and are considered as typical of the massive group. 
The laminations are generally apparent, but in many instances 
the rocks present a well-nigh massive appearance. Running 
like broad black ribbons in directions generally vertical, across 
the exposed faces are numerous dikes varying in width from a 
few inches to forty feet. Fifteen were noted in Storm King, 
six in Cro’ Nest. 
These dikes are very uniformly holocrystalline aggregates of 
hornblende, augite and plagioclose as principal minerals, with 
subordinate magnetite and apatite, and occasionally a little biotite, 
orthoclase and quartz. The hornblende and augite are gener- 
ally associated, but there are instances in which each appears 
alone with the other minerals mentioned. The augite appears 
alone especially in those dikes whose wall rock is most broken and 
contorted. The hornblende is of the common brown variety 
strongly pleochroic. It is never in well-bounded crystals, but 
always in irregular masses, whose external shapes are conditioned 
by their neighbors. It frequently contains included the apatite 
and magnetite, and the masses vary from 0.5 mm. to 1.0 mm. in 
width, but are relatively somewhat longer. In the more altered 
specimens the hornblende tends to bleach out to a green variety. 
The augite is in the same irregular masses of light green color, 
and contains the same inclusions, but on the whole is less prone to 
do so. The biotite is far less abundant than the two just men- 
* The School of Mines Quarterly, vol. ix., p. 34. 
