OF CENTRAL CANADA PART III. 153 



hydraulic or water lime. Beds of this kind occur at Thorold, Cayuga r 

 Loughboro', Kingston, Hull, Quebec, and other localities. See 

 Part V. 



Conglomerates consist of rounded pebbles or masses of quartz, sand- 

 stone, &c., cemented together, or imbedded in a paste of finer sand- 

 stone, or other rock substance. They are often known as " Pudding 

 stones." Examples are not uncommon amongst our Silurian and 

 other strata. 



Breccias consist of angular masses or fragments of rocks, cemented 

 together most commonly by calcareous matter. Whilst Conglomerates* 

 frequently contain imbedded water-worn materials derived from dis- 

 tant sources, true breccias are necessarily composed of detrital matters- 

 derived from neighbouring localities. 



Trap-tufas are of comparatively rare or local occurrence. They are 

 made up of materials derived from the wasting of trap or greenstone 

 rocks, and are mostly of a green colour, weathering red. Their tex- 

 ture is generally more or less uniformly fine-grained ; but some occur 

 as conglomerates and breccias, as on the north-eastern shore of Lake 

 Superior, and elsewhere. 



The other rock-substances enumerated above Coals, Gypsum, Rock- 

 salt, and Bog Iron Ore occur only here and there as stratified rock 

 deposits. For descriptions and Canadian localities, see Part I. 



(2) FORMATION OF SEDIMENTARY ROCKS. 



The manner in which the ordinary sedimentary rocks, sandstones, 

 shales, &c., have been formed, or built up as it were, is rendered clear 

 by the observation of certain natural processes still in action. We 

 find, for example, at the present day, that sediments of various kinds 

 are constantly carried clown by streams and rivers into lakes and seas, 

 and are there deposited. We fincj, moreover, that the cliffs of many 

 sea and lake coasts are being continually abraded and washed away by 

 the action of the waves. Observation shows also, that the sedimentary 

 matters thus obtained, are always deposited or arranged in regular 

 layers or beds, and that they frequently enclose shells and sea-weeds, 

 together with bones and leaves drifted from the land, and other organic 

 bodies. Hence it is now universally admitted, that, with the excep- 

 tion of certain limestones and dolomites, beds of rock-salt, gypsum, 

 coal, and some other chemical or organic deposits of small extent, all 

 the sedimentary rocks have been formed directly out of pr viosuly- 



