Dikes of the Hudson River Highlands. 697 
In some cases these dikes are parallel to the laminations of the 
enclosing gneiss or syenite, in other cases cut across them. Many 
would seem at first sight to be interbedded masses, and the writer 
would confess himself inclined at first to take the view that if 
these mountains are to be considered metamorphosed sediments, 
then these dikes represent strata of composition different from the 
remainder, There are, however, illustrations, as stated above, of 
unconformability, and not only that, but of two separated and 
unconformable branches joining above while separated below. (Spec. 
82 and 33, two dikes each twelve feet wide just south of Cro’ 
Nest flag station.) They are, therefore, esteemed of undoubted 
intrusive origin. 
he dikes are not infrequently faulted by feldspathic segrega- 
tions. In Storm King, above the paving stone quarry, six, from 
four inches to eighteen inches wide, are exposed, four of which are 
faulted by such a segregation. The inference from this is that 
the dikes are of great geological antiquity. These feldspathic 
masses consist chiefly of very coarsely crystalline orthoclase. 
Such a mass we know would form only under high pressure and 
great heat,’ and indicates the changes through which these rocks 
have passed since the dikes were intruded. The geological date of 
the metamorphism, if such it were, which gave these syenites or 
gneisses their present form, it is not easy to state, but from the 
comparatively unchanged condition of the strata lying against them 
to the north at Cornwall, it must have been before the Hudson 
River Period of the Lower Silurian. In the writer’s opinion the 
dikes were intruded in Archæan time, and have experienced the 
same influences which have given the gneisses their bedded character. 
It cannot be affirmed that the dikes themselves are metamorphosed 
from their original structure, but it is interesting to note that they 
exhibit even in their narrowest examples a perfect holoerystalline 
structure, nor is any amorphous or porphyritic matter to be detected. 
e infer from this either that they were intruded between highly 
heated walls, and that they cooled slowly and under pressure like 
a plutonic rock (Tiefengestein); or else that subsequent metamor- 
* Hautefeuille, Comptes Rendus, 1877, t. Ixxxv., p. 952, idem. t. XC., 
aah 830. Friedel et Sarasin, B. Soc. Min., 1879, p. 158, and 1881,. 
