38 
Rocks of Tabletop 
The Tabletop batholith consists in the main of medium-grained, 
reddish granite made up of orthoclase, plagioclase, quartz, and brown or 
green biotite, often accompanied by hornblende, and a small amount of 
magnetite. It seems to pass into a finer-grained, paler granite in places, 
and to be cut by the latter as dykes in other places. This rock, which 
shows a tendency to porphyritic structure — enclosing fairly large crystals 
of strikingly zonal plagioclase — and contains very' little of the dark minerals, 
probably suggested the term “trachytic granite” to the earlier geologists. 
A fine-grained, flesh-coloured rock collected as granite on Northeast dome 
turns out to be really diorite, since most of its feldspar is plagioclase with 
a low extinction angle. There is little quartz, and the plentiful dark 
ingredient is hornblende with a brown and green pleochroism. 
In many places patches or fairly well-defined blocks of dark grey or 
green rock occur in the granite, and on the north shore of Cote lake there 
are apparent transitions between granite and greenstone. The specimens 
collected are unfortunately' badly weathered. A medium variety, looking 
in the hand specimen like syenite, is really a quartz diorite, having as 
dark minerals both biotite and hornblende, and a few large crystals of 
titanite. Darker coloured specimens, too much weathered for satisfactory 
microscopic study, show a tendency to ophitic structure, and one collected 
because it seemed filled with small cavities, suggesting a vesicular lava, 
proves in fresher parts to contain little masses of pyrite. 
How these more basic rocks are really related to the prevalent granite 
where the transitions occur is not very certain, though there are places 
where they are enclosed as sharp-edged xenoliths, and, therefore, must be 
older. 
In addition to the rocks just described, which were collected in place, 
there are many boulders at the Forks of Ste. Anne river, of which specimens 
were taken, some of typical granite like those referred to, but others of 
different kinds, particularly quartz porphyries and porphyrites. The 
quartz porphyries are often of the same colour as the granites, and the 
matrix is microgranitic, enclosing phenocrysts of orthoclase, or more often 
an acid plagioclase, with fewer phenocrysts of quartz. 
The porphyrites examined are weathered diabase porphyrites with 
large and well-formed plagioclase crystals. A medium-grained diorite was 
collected also, composed chiefly of oligoclase, hornblende, arid brown 
biotite,. with magnetite and apatite needles as accessory minerals. 
The rocks just mentioned probably belong to the Tabletop eruptives, 
though their definite source is unknown. Unless the basic rocks mentioned 
above were earlier products of the original magma and not merely frag- 
ments of older rock caught up in the irruption of the granite the amount 
of differentiation in this mass was slight. Nothing in the nature of peg- 
matite as a final stage in differentiation has been observed. 
As might be expected a mass of slowly cooling granitic magma of such 
magnitude has had considerable effect in the metamorphosing of the 
adjoining sedimentary rocks, illustrated in the formation of schists and of 
quartzite along the western side of the mountain group. 
Certain highly metamorphosed schists found in boulders at the Forks 
may, however, belong really to the belt of Precambrian indicated on the 
geological map. They' include finely fibrous green schist consisting of 
