'OF CENTRAL CANADA PART III. 151 



^Sandstones, sands, and gravels or arenaceous rocks. 



Clays and clay-slates or argillaceous rocks. 



Limestones and Dolomites or calcareous rocks. 



Conglomerates and Breccias : rocks of variable composition (see 

 below). 



Trap tufas : stratified deposits formed out of materials derived from 

 the denudation of trap and greenstone rocks. 



Hock matters of carbonaceous origin, as the different kinds of coal. 



To these may be added a few other substances of subordinate occur- 

 Tence, as gypsum, rock-salt, and bog-iron ore. 



Sandstones are nothing more than beds of consolidated sand. They 

 .are of various colours, but chiefly present dull shades of yellow, red, 

 brown, or green, and some are nearly pure white. The colouring 

 matter is either sesquioxide of iron, or, in the case of the greenish 

 varieties, a silicate of the protoxide. The harder and purer kinds, 

 as some examples of our " Potsdam sandstone," are called quartzose 

 sandstones. In other kinds, a certain amount of carbonate of lime 

 is present, cementing together the component grains of sand, and 

 thus forming calcareous sandstones. For special Canadian localities 

 of these and other rocks mentioned under this division, consult Part 

 V. Certain siliceous rocks, called "tripoli" and (erroneously) 

 *' infusorial marls," are formed almost entirely of remains of diatoms, 

 .microscopic vegetable forms of low organization. (See Part IV.) 



Clay Slates are merely consolidated clays. They have a fissile 

 structure, and are mostly of a grey, greenish, brown, or black colour 

 the dark tints being chiefly derived from the presence of finely dis- 

 seminated carbonaceous or bituminous matter. Clays are also of 

 various colours, as white, greenish, yellowish, blueish, black, and red. 

 Those which contain little or no iron become white or pale yellow on 

 ignition. Many clays are highly calcareous ; others, bituminous, &c. 

 The term shale is often applied to fissile consolidated clays ; but this 

 term, it must be remembered, is applied equally to fissile or slaty 

 limestones and sandstones. When the term is used, therefore, the 

 .kind of shale should also be signified : as an argillaceous shale, an 

 arenaceous shale, and so forth. Bituminous shales, as regards their 

 mineral base, may be also arenaceous, calcareous, &c. 



Limestones and Dolomites are principally, perhaps, of chemical for- 

 anation. Water containing free carbonic acid (derived from decay- 



