No. 3.] NATURAL HISTORY SOCIETY. 169 



Affaiu, alotiu' certuiu lines of the eartlis crust, the beds 

 deposited by water have been folded and crushed together, prob- 

 ably by the contraction of the earth's shell in cooling, and along 

 these lines they have b?en changed, in the way of hardening and 

 becoming crystalline or in being chemically recompounded — 

 alterations which are usually known as metamorphic. But still 

 further, some kinds of deposit are much more liable to such 

 metamorphic changes than others. More especially the beds of 

 igneous origin, from their containing abundance of basic matter, 

 as well as of silica, very readily change under the influence either 

 of heat or water, becoming it may be highly crystalline, or having 

 new mineral substances formed in them by new combinations, or 

 on the other hand, when acted on by water, combining with it 

 and forming hydrous silicates. 



One other curious coincidence it is necessary to mention. — 

 It is where the greatest deposits of sediments are going on along 

 coasts or in the course of currents, that crumpling and bending 

 of the crust are most likely to occur, and igneous ejections to be 

 thrown out ; and conversely, where igneous ejections are piled up, 

 coasts may be forming or currents deflected, so as to cause at 

 these points the greatest deposit of sediment. 



These considerations are sufficient to shew the true value of 

 mineral character, first as a means of distinguishing rocks of 

 different nature and origin, and secondly of separating rocks of 

 different ages within limited localities ; with its entire worthless- 

 ness when applied to distinguish the ages of beds in widely sepa- 

 rated localities. There are in America rocks as widely apart in 

 time as the Huronian of the East and the Carboniferous of the 

 West, which are scarcely distinguishable in mineral character : 

 there are rocks of identical ase. as for instance the Lower Silurian 

 of New York and Western Canada and that of Nova Scotia and 

 of Cumberland, which are as unlike in mineral character as it is 

 possible for rocks of the most diverse ages to be. 



But can we trust implicitly to stratigraphy ? Certainly, when 

 we find one rock directly superimposed on another we know that 

 it is tlie newer of the two. But when we find old rocks slid 

 over new ones by reversed faults, when we find sharp folds over- 

 turning great masses of beds, and when we find portions of beds 

 hardened, altered, and become more resisting, standing up as 

 hills in the midst of the softer materials, perhaps of the same age, 

 which have been swept away from around them, then we have 

 the real difficulties of stratigraphy. 



