OF CENTRAL CANADA PART III. 179 



base of this, or of ordinary granite, numerous crystals of feldspar are 

 distributed, when the rock is known as porphyry, or, better, as por- 

 phyritic gra die or porphyritic felsite (Fig. 97). The imbedded crys- 

 tals often show the twin or compound 

 structure so common in feldspathic sili- 

 cates. The term " porphyry " (from nop- 

 <pvpa), as the name would indicate, was 

 originally applied to rocks of this kind, 

 in which either the base or the imbedded 



crystals presented a deep-red colour ; but it is now bestowed conven- 

 tionally on all rocks containing distinct crystals of feldspar or other 

 minerals. We have thus porphyritic granites, porphyritic syenites, 

 porphyritic trachytes, porphyritic greenstones (the original por- 

 phyry having been probably one of these latter), porphyritic lavas, 

 &c. Finally, as regards other granite varieties (to many of which 

 special names of uncertain or merely local application have been use- 

 lessly given), it may be observed that the mica of ordinary granite 

 is occasionally replaced by talc, giving rise to talcose granite (the 

 Protogine of some authors), or is accompanied at times by hornblende, 

 the rocks in the latter case being known as syenitic or hornblendic 

 granite. By the gradual diminution of quartz, the granites proper 

 pass into granitic trachytes, described below ; and they are repre- 

 sented in the metamorphic series by gneiss and gneissoid rocks gen- 

 erally, into which also they appear locally to merge. 



Examples of intrusive granite occur in most parts of the large area 

 occupied by the Laurentian rocks of Canada (see Part V.). Porphy- 

 ritic felsite, in which the base is mostly dull-red or greenish-black, and 

 the imbedded feldspar crystals red or pink, is seen in connection with 

 a large mass of syenite in the Township of Grenville on the Ottawa. 

 This variety, sometimes termed Orthophyre, is scarcely perhaps a true 

 granite, but as it contains free quartz it must be referred convention- 

 ally to the granitic series. A broad dyke or vein of graphic granite 

 (consisting of quartz and orthoclase-feldspar) is described in the 

 Reports of the Geological Survey as occurring on Allumette Lake, 

 north of Pembroke, and other examples of a similar character have 

 been recognized in the neighbouring Township of Ross. Veins of both 

 ordinary and quartzo-feldspathic granite, in some cases holding crystals 

 of tourmaline or schorl, occur also more or less abundantly, in St. 



