E. 8. Willbowrn—Alteration of Granitic Rock. 443 
rock appear to be identical with the few larger flakes of mica in 
the quartz-mica rock. 
The dark-blue bodies meneioned above consist of quartz and blue 
tourmaline. The quartz occurs as irregular grains of the same size 
as those in the white quartz-felspar or quartz-mica rock which 
encloses the bodies. 
The tourmaline occasionally shows crystal outline, then occurring 
as tiny prisms, but usually it occurs as shapeless grains, one of which 
may enclose two or three of the quartz grains. The dividing line 
between the quartz tourmaline bodies and the surrounding white 
rock is very irregular, particularly in the quartz-felspar rock, The 
tourmaline seems “to be replacing muscovite in the quartz-muscovite 
rock, and felspar in the quartz-felspar rock. The difference in the 
appearance of the quartz-tourmaline bodies is very striking in hand- 
specimens of the two rocks, for the bodies in the quartz-muscovite 
rock appear to have a sharp outline, and their shape is roughly 
spherical. They weather out in a conspicuous fashion, and in places 
can be found lying about on the rock surface. The bodies in the 
quartz-felspar rock have an irregular branching outline. 
Sometimes the bodies are more or less regularly distributed 
throughout the mass of the white rock (quartz-muscovite or quartz- 
felspar), but in some exposures there is an arrangement along definite 
parallel planes. One specimen of quartz-mica rock collected from 
the east end of the hill range shows a large number of the bodies 
partially amalgamated to form a plate, so that a cross section has the 
appearance of a string of beads. Under the microscope these bodies 
are seen to have the same structure as those already described, and the 
plate, although not unlike a vein, is seen to be very different from certain 
true quartz-tourmaline veins which were observed in several parts of 
the hills, penetrating the quartz-muscovite rock. These veins are 
minute with a thickness of only about + to 4 of an inch. They 
‘consist of long prisms of blue tourmaline set in a mosaic of clear 
quartz. The veins are of a very much coarser grain than the parent 
rock. They have sharply defined walls. Sometimes a prism of 
tourmaline was observed not to end abruptly at the wall of the vein, 
but to penetrate for a short distance into the quartz-mica rock. As 
soon as it left the vein, however, it lost its sharp crystal outline and 
enclosed the quartz grains of the quartz-mica rock, taking the place 
of the interstitial flakes of muscovite. 
In exposures immediately east and west of the beacon hill, the 
tourmaline bodies are very small in size, being no bigger than a pin’s 
head. More often than not they are arranged along definite planes, 
otherwise there is no distinction between them and the larger bodies. 
Two small exposures on opposite sides of the beacon hill show a 
rock of a shghtly coarser grain than that already described. More- 
over, no tourmaline can be seen in the hand-specimen. Under the 
microscope it is seen to be porphyritic, the phenocrysts being bleached 
biotite and quartz with the irregular outline characteristic of the 
corroded quartz phenocrysts in porphyritic rocks. In addition there 
are areas of secondary mica whose form suggests that they are altered 
felspar phenocrysts. The biotite contains needles of rutile associated 
