GABBROS OF EAST SQOKE AND ROCKY POINT. 
5 
compositions, suddenly changing from one into another, and 
bewildering to the observer by reason of their number and the 
rapidity with which they succeed one another. Owing to this 
complexity, it is impossible to map the different types even on 
the large scale map that accompanies this report. An attempt 
has been made to indicate the positions of a few of the larger 
masses, but the boundaries of these are only approximate. 
This rock complex is cut by a great number of dykes and 
igneous veins, which vary widely in composition. The earliest 
are of nearly the same composition as the normal gabbro, and 
are often porphyritic; a few of them contain olivine, but the 
majority do not. In the Rocky Point mass more acid types are 
found, younger than some at least of the gabbroid types. They 
approximate the granite in composition, and are supposed to 
represent part of thfe granitic differentiate of this magma. They 
ate characterized by the presence of free quartz, sodic feldspars, 
hornblende, and titanite. These dykes, though often large, are 
never porphyritic. The dykes and older rocks are cut by a great 
number of replacement veins of all sizes, from a fraction of an 
inch in width up to 2 feet. These veins are composed of long- 
bladed hornblende crystals, without, as a rule other constituents 
than magnetite. Frequently large zones of homblendite, up to 
250 feet in width, are found. These do not appear, however, to 
have been formed by the deposition of homblendite in a single 
large fissure, but rather by the ascent of solutions through a 
multitude of closely spaced slip planes of a large shear zone, 
with consequent thorough and rapid alteration of a large mass of 
country rock. 
Still younger than these homblendite veins, and cutting them,, 
are large numbers of aplite veins. They are always of small 
size, rarely over an inch in width, and form both replacement 
veins and true fissure fillings. They are composed of quartz 
and albite or oligoclase-albite, with very little ferromagnesian 
minerals. 
