432 A POPULAR EXPOSITION OF THE 
the base or imbedded crystals were of a red colour; but it is now 
conventionally bestowed on all rocks containing distinct crystals of 
feldspar or other minerals. Thus, we have porphyritic granite, por- 
phyritic trachyte, &c. Occasionally, the mica in granite is replaced 
by tale, giving rise to Talcose Granite. Sometimes, also, the mica 
dies out, when a granitic mixture of quartz and feldspar results. 
This has been called Pegmatite. 
Examples of intrusive granite occur amongst the strata of the 
Laurentian and Huronian series in the Lake Superior region and on 
the north shore of Lake Huron, and elsewhere, but apparently in no very 
prominent masses; although veins composed of quartz and feldspar, 
or of quartz alone, are of exceedingly common occurrence throughout 
the entire area occupied by the gneissoid Laurentian rocks. Fig. 53 
52 is a sketch of some quartzo-feldspathic 
i veins in gneiss, near the right bank of 
the river Severn, C. W. In the more 
modern metamorphic district south of 
the St. Lawrence, however, granitic mas- 
ses (which appear to pass into granitic 
trachytes or diorites) constitute the 
Megantic mountains, and occur also in force in Hereford, Stanstead, 
and other townships of that district. (The localities cited by Sir 
William Logan, in his Esquisse Géologique du Canada, comprise : 
Stanstead, Barnston, Hereford, Marston, Megantic Mountains, Wee- 
don, Winslow, Stafford, and Lambton.) 
Syenite.—This eruptive rock is composed of a granitic mixture of 
quartz, feldspar, and hornblende, the latter being green or black in 
colour. When mica is also present, the rock becomes syenztie granite ; 
and when the quartz grows gradually less and less abundant, there is 
a transition into granitic diorite or greenstone. .Some syenites are of 
a red colour from the prevalence of red feldspar, and many syenites 
are porphyritic. Intrusive syenite occurs amongst the Laurentian 
rocks in various localities. An enormous mass of this rock, covering 
an area of thirty square miles, is cited by Sir William Logan, as oc- 
curring in the townships of Grenville, Chatham, and Wentworth, in 
Argenteuil County, on the Ottawa. 
Granitic rocks frequently become converted, by the decomposition 
of the feldspar, into white or light-coloured clays, largely used, under 
the name of Kaolin, in the manufacture of porcelain. 
