GABBRGS OF EAST SOOKE AND ROCKY POINT. 
7 
thus from a type made up of 60 per cent feldspar, 15 per cent 
pyroxene, and 25 per cent olivine, the rock may pass, without 
change in pyroxene content, through types with decreasing 
olivine and increasing feldspar, into a rook with 85 per cent feld- 
spar a$id little or no olivine ; or with constant olivine content, by 
decrease of feldspar and increase of augite, into a basic type 
carrying not more than 40 per cent feldspar with 35 per cent 
pyroxene; or the olivine content may become small or zero, and 
augite gabbros result, varying all the way from anorthosites 100 
per cent feldspar to basic types with 60 per cent of pyroxene. 
Singularly enough, the composition of the individual minerals 
remain uniform, or approximately so, during these variations in 
proportion. A .single example may be cited. On Bentinck 
island, south of Rocky point, four parallel gneissic bands of gab- 
bro occur. The proportion of feldspar varies from about 50 per 
cent in the most basic band to about 90 per cent in the most 
feldspathic, but its composition in all four bands is Ab 20 An g0 . 
The common olivine gabbro is equigranular, with an average 
grain of 1 to 2 mm. The feldspar in it tends to form squarish 
or rectangular crystals whose length is rarely more than twice 
the breadth. The augite is interstitial to the feldspar, and cf 
much the same general shape. Three types varying from the 
normal texture are found frequently enough to be worthy of 
separate description. 
Type A, termed pegmatite in the field because of its coarse 
grain, possesses the same equigranular texture and mineral com- 
position as the common olivine gabbro, but the component min- 
erals attain a diameter of 4 to 5 mm, on the average. In addi- 
tion, the feldspars as a rule are zonally banded. Another, less 
common, and much more coarsely crystalline rock which may be 
classed with these “ pegmatites ” is occasionally found in rounded 
masses 6 feet or more in diameter included in rock of ordinary 
grain, into which it grades rapidly. In these masses the average 
grain approximates 15 to 25 mm. 
Type B is a rock of ordinary grain, but is characterized by 
rather long prisms of augite and long laths of feldspar. The 
