He
distinguished between two series of pegmatitic developments, and
indicated that they were marked by a succession of intersections. The
oldest series is the most extensive, and lies in the foliae, or leaves,
of the inclosing mica rock and assumes the latter's strike. The later
or younger series cuts the schists in all directions and all
inclinations. Dr. Julien avers that all the coarse granites of the
island have been originally veins "segregated from a magna or
igneous-aqueous emulsion," which probably means that these granites
have crystallized from a viscous or semi-fluid rock paste, however that
rock paste originated. In this crystallization they assumed a vein
structure showing correspondent deposits on the two walls, or a comb
structure with the less acid minerals on the outside and the more acid
minerals at the center of the vein, with often also a concentration of
minerals of rare elements (xenotime, monazite, allanite, etc.), in
smoky quartz near a central suture.
But
this original vein structure has been powerfully compressed in the
mountain-making movements of the schists around it, and there has been
Assuring, faulting, crushing, shearing " with development of aplite (a
granite dike) refusion, and development of new phenocrysts (granite
porphyry) and the generation of reaction borders outside of each wall
of vein," or, in other words, the vein granites have assumed the nature
and function of intrusive granites, and in the surrounding rock their
heat has developed accessory minerals. Flow-age occurred and the
crushed vein matter of the granite became dike-like along the plane of
the veins. But Julien insists that " in the most characteristic dikes
the vein structure is rarely, if ever, completely obliterated." Dr.
Julien's diagnosis is certainly not startlingly at variance with that
given above, but presents the further conception of the vein-like
nature of the New York Island granites, when first formed.
Such
a view does not apply to the fine-grained granites, as the large
development on the west side in the fifties, which has all the
appearance of an intrusive rock, a welling out of a pasty or liquid
mineral body.
