Common RocJcs of the British Frovlnces. 199 



the granites are called Plutonic rocks, because tliey most abound and come 

 up from his dominions to us upon the surface. 



Granite is not stratified, but the rocks we are next to consider, although 

 composed of the same ingredients, are disposed in regular layers from one 

 Inch to several feet in thickness, and it is these with which the student of 

 Canadian geology will most frequently meet as they extend over the whole 

 of the Laurcntian country. 



They consist of the diScrent varieties of gneiss and schist. 



1st. Gneiss is composed of quarts, felspar, and mica, stratified or arranged 

 'in regular beds. 



2ad. Syenitic gnem consists of the same materials, but with the addi- 

 tion of hor?iblende. This rock largely abounds in Canada, constituting the 

 principal portion of the Laurentian formation. The principal difference 

 Ijetween gneiss and granite is, that the latter is a rock of igneous origin, 

 while the former is composed of materials deposited upon the bottom of the 

 ■ancient oceans — and there consolidated. The Laurentian formation was 

 therefore accumulated in a sea of still greater antiquity than that in which 

 the Potsdam Sandstone was deposited. Boulders or beds of Oneiss may in 

 .general be recognised by their striped appearance. They often consist of 

 alternate thin layers of quartz, felspar, and mica, and these minerals being of 

 ■different colours the sides of the rock which j^resent the edges of the 

 respective laminas exhibit numerous stripes of white and black, or of lighter 

 .and darker colours. There is thus a sort of double stratification of gneiss. 

 In the first place the formation is divided into strata, from one inch to three 

 or four feet, and even twenty feet in thickness, totally separated from each 

 other, and in the second place each bed consists of numerous laminae or thin 

 leaves, from one eighth of an inch or less to several inches in thickness, 

 'blended togethei\ 



If the observer can see that a piece of rock is composed of quartz, 

 felspar, and m.ica, and also that these ingredients ai'e not uniformly mixed 

 throughout, but arranged in alternate thin leaves, then he may be certain 

 the specimen is gneiss in some one of its varieties. 



Syenite is a name given to a variety of rock obtained from the quarries 



■of Syene, in Egypt. It consists of quartz, felspar, and hornblende, but no 



mica. When hornblende is present in granite, the rock is called Syenitic 



granite, and when this mineral also occurs in gaeiss, it is called Syenitic 



gneiss. 



Ie the Laurentian formation the Syenitic gneiss is often of various 

 shades of grey, and much resembles grey granite. Great masses may often 

 be seen of a red colour, owing to its being largely composed of red felspar. 

 Other varieties are almost black, from the great proportion of dark 

 •blackish green, hornblende, or mica they contain. These latter vai'ieties also 

 j)ass into schist, which is thus described by Sir Charles LyelL 



'• Hornblende schist is usually black, and composed principally of horn- 

 -Mende, with a variable quantity of felspar, and sometimes grains of quartz. 

 When the hornblende and felspar are neai'ly in equal quantities, and the rock 



