OF CENTRAL CANADA PART III. 165 



levels occupied by a displaced bed are 

 sometimes only a few inches, and at 

 other times upwards of a thousand feet, 

 apart. At the first formation of a fault 

 or slip, an escarpment or terrace of 

 greater or less height must necessarily 

 have been produced ; bitt in very few 

 cases (if in any case unconnected with 



x . Fig. 91. 



existing earthquake phenomena) is any- 

 thing of this kind now observable, the ground having been levelled 

 <lown at some after period by the agency of denudation. In moun- 

 tainous districts the fracturing of strata has sometimes given rise to 

 narrow gorges or so-called " valleys of dislocation," but most of these 

 .have been subsequently enlarged by the atmospheric disintegration 

 of the surrounding rocks, and by the streams or torrents of which 

 they usually form the channels. In most faults the displaced beds 

 have slipped downwards, and have thus been brought into a lower 

 position than that which they originally occupied ; but occasionally, 

 ;as in the so-called " reversed faults," they have been forced upwards, 

 so as in some instances to overlie the other beds. 



(d) Metamorphism. Many strata afford undeniable proofs of 

 Laving been greatly altered, as regards texture and other mineral 

 characters, from their original sedimentary condition. In many in- 

 stances, indeed, the original composition of the rock appears to have 

 been changed. Strata thus affected, are commonly known as rneta- 

 morphic or altered rocks. In some cases a passage can be traced from 

 the altered into the unaltered parts of the rock; but, frequently, where 

 rocks have been subjected to this action, the alteration has extended 

 over wide areas, and has been more or less complete. It consists most 

 commonly in the assumption of a crystalline structure, and is very 

 generally accompanied by the presence of crystallized minerals and 

 other indications of chemical action. 



In numerous instances, metamorphism, on a limited scale, has 

 evidently resulted from the direct intrusion of eruptive rock matters 

 amongst sedimentary formations. Where trap dykes or masses of 

 granite, for example, have been thrust up through fissures in ordinary 

 strata, the latter are seen in many cases to have been more or less 

 altered around the points of contact, as though by the agency of 

 intense heat, or by that of steam or other gases acting under pressure. 



