PETEOGEAPHY OF ONTAEIO. el 



Gneiss. 



Grueiss differs from granite only in the parallelism of some of its constituents, gener- 

 ally the mica and hornblende, but sometimes also the quartz. When indistinctly schistose, 

 it is often indistinguishable from granite when occurring as boulders. Gneiss is probably 

 the commonest material of our boulders, with the exception of limestone, but needs no 

 special description, since almost everything said of granite applies here also. To the 

 varieties described under the head of granites we may add two more for the gneisses, 

 garnetiferous gneiss and tourmaline gneiss. 



Garnetiferous Gneiss contains flesh-coloured garnet in so great an amount as to be no 

 longer an accessory mineral. The garnets generally accompany hornblende and seem 

 partially to replace orthoclase in a few cases. In one specimen from Bowmanville the 

 crystals of garnet are sometimes as much as an inch in diameter, but are far from hand- 

 some. 



Tourmaline Gneiss. — In a few instances black tourmaline (schorl) replaces the mica, 

 having its nine-sided prisms arranged with their longer axes all in the same direction. 

 The dichroism (light brown and black) is very characteristic. 



The gneisses vary exceedingly in structure, some being scarcely at all schistose and 

 others almost as much so as mica schist. Some are porphyritic (orthoclase porphyroide) 

 and contain large felspar crystals, often Carlsbad twins. The augengiieiss structure pre- 

 vails now and then, large individuals or masses of felspar of a lenticular shape wedging 

 apart the ordinary schistose layers. 



Of Clastic Origin. — A few specimens give hints of a clastic origin in the granular look 

 of the quartz ; and one small boulder coming from Marmora is very evidently clastic, 

 since it contains large, well rounded pebbles of quartzite and gneiss, recalling the augen 

 mentioned before. The layers of true gneissic quartz, felspar and mica, adjust themselves 

 in quite the same way as around the augen. 



Hulleflinta. 



By a diminution of the grain, so that the individual minerals are undistinguishable 

 by the eye, gneiss may pass into hiiUellintas, compact rocks shewing traces of schistose 

 structure or evidences of stratificatiou in layers of slightly varying color. The hiillellintas 

 have the same relation to gneiss that felsite bears to granite. Where there is a tendency 

 to the separation of larger masses or crystals of quartz and felspar they approach the group 

 of orthoclase porphyroidcs ; but true porphyroides seem scarcely to occur in our drift. 



Macroscopic. — These rocks are in general flesh-coloured or reddish grey and exceedingly 

 hard. Thej' form a considerable part of the smaller boulders. 



Microscopic. — Under the microscope their structure is either that of a micro-gneiss 

 or resembles that of felsite but iisually with great A^ariations in the grain. When the 

 minerals can be distinguished, they appear to be quartz, orthoclase or a little microline 

 and plagioclase. Magnetite also is almost constantly found. A little biotite, muscovite 

 or hornblende may occur also, but not at all with the constancy of these minerals in 

 gneiss. As accessory minerals we may mention garnet occurring as minute but perfectly 

 formed rhombic dodecahedra, black tourmaline which occurs in one specimen as tiny 



