220 
Collie—Geology of Conanicut Island , R. I. 
ularly distributed throughout the siliceous matrix. The quartz 
grains vary in diameter from .04 to 1 cm. The ottrelite 
crystals are 4 or 5 cm. in length. They contain included within 
their borders patches of the ground mass, flakes of iron oxide 
and graphite. The edge of the crystals is commonly bordered 
by interlocking quartz grains, which do not orient with each 
other. These crystals are fresh and contain few inclusions, 
thus being strongly contrasted with the grains of the ground 
mass, which are full of inclusions. The former are of second¬ 
ary origin and it is doubtless the development of this secondary 
quartz that gives the rock its quartzite character. 
The ottrelite plates do not lie in any given plane, as for in¬ 
stance that of schistosity, but are indefinitely placed in any 
position. The character of the formation, the inclusion of por¬ 
tions of the ground mass, the orientation of every portion of 
the large plates indicate that the ottrelite was formed seconda¬ 
rily subsequent to the formation of the schistosity even, for it 
must have been oriented with this structure if formed prior to or 
during its formation. As the ottrelite formed and spread it 
included the ground mass within its borders. The secondary 
quartz was formed later since the direction of its growth is 
frequently determined by the edge of the ottrelite crystal. The 
ottrelite is frequently altered to chlorite. The latter in turn 
becomes opaque and dull colored, giving rise to a substance resem¬ 
bling kaolin. The pieochroism of the ottrelite is yellow green 
parallel to c and olive green parallel to a. As in most of the 
metamorphic rocks garnets abound in the ottrelite schist; they 
are of pale rose color and sometimes have a slight double re¬ 
fraction. They are usually changed to chlorite and surrounded 
by a zone of iron oxide. As stated above, there is a remarka¬ 
ble contrast between the core of ottrelite bearing schist and the 
surrounding schists. The ottrelite schist resembles an included 
boulder, for there is no gradual transition from black schist to 
ottrelite schist, the contact is sharply defined. 
In many cases the waves have worn away the soft schists and 
left the ottrelite beariug cores outstanding which serves to 
heighten the impression that they are erratic boulders. In spite 
of this appearance they are doubtless formed in situ as lenticular 
