OF CENTRAL CANADA PART III. 



149 



In different localities, as a general rule, the rocks which form the 

 surface of the ground, or which become visible to us on the sides of 

 cliffs and river-banks, in quarries, railway cuttings and the like, are 

 more or less distinct in composition and other characters. This must 

 be familiar to the most casual observer. Thus, around the Falls of 

 Niagara, and extending far and wide across that section of the Pro- 

 vince, we find vast beds of dolomitic or magnesian limestone present- 

 ing several varieties of texture. About Hamilton and Dundas, with 

 other rocks, ferruginous shales and beds of red marl and grey sand- 

 stone are seen. At Toronto, our rock-masses consist of layers of 

 gravel and clay, overlying grey and greenish sandstone-shales. Near 

 Collingwood, and again at Whitby, we observe dark-brown, highly- 

 bituminous shales, containing the impressions of trilobites and lin- 

 gulae (see Part IV.), often in great numbers. At Kingston, we meet 

 with limestone rocks differing from those of the Niagara district, and 

 giving place, as we proceed north and east of the city, to beds of 

 crystalline rock of granitic aspect, geologically known as Gneiss. 

 Some of the " Thousand Islands " consist of very ancient sandstone 

 resting on gneiss. At Montreal, with beds of limestone, &c., we 

 see, in the picturesque Mountain, a dark, massive or unstratified 

 rock, a variety of the Trappean series, more or less closely allied to 

 the lavas of volcanic regions ; and rocks of a similar kind occur 

 largely on the north shore of Lake Huron, and around Lake Su- 

 perior, as well as in the Eastern Townships and other parts of 

 Canada. 



These examples are sufficient to shew the diversity which prevails 

 with regard to the rock-matters of comparatively neighbouring locali- 

 ties. But if we look, not to the mineral characters of rocks, but to 

 their general conditions of occurrence, by which their respective 

 origins or modes of formation are indicated, we may refer them to 

 two leading groups or sub-divisions, connected by an intermediate 

 group, as in the following scheme : 



SEDIMENTARY ROCKS or Ordinary Stratified-Formations. 



METAMORPHIC ROCKS or Stratified Crystalline-Formations. 

 ERUPTIVE or UNSTRATIFIED ROCKS. 



Sedimentary strata, comprising ordinary sandstones, limestones, 

 &c., consist of detrital or other materials, collected, and arranged in 

 more or less regular layers, by the action of water, as described below. 



