334> ON THE APPARENT DISLOCATIONS [Ch. XV. 



These subordinate movements are said to be indicated by 

 the curvatures and contortions of the strata, and by the 

 various phenomena exhibited by granite-veins, by dykes of 

 porphyry, trap, and similar rocks, and by mineral and 

 metalliferous veins. In the narrow limits of this work, these 

 important topics cannot be fully detailed ; it must therefore 

 suffice to notice those facts only which bear more immediately 

 on the question. 



It is generally admitted that the layers of rocks may 

 attain a certain degree of curvature, by deposition on an 

 undulating surface s such as the exterior of the earth actually 

 presents. But this cause has its limits, and is in no wise 

 applicable to cases of complicated flexures and contortions, 

 which so frequently occur in every country. These appear- 

 ances were once thought to be confined to the older or pri- 

 mary strata ; but it is now known that, though most frequent 

 in gneiss and mica-slate, they do exist not only in every 

 secondary group, but also in the tertiary formations. The 

 precise manner in which these curvatures have been pro- 

 duced has given rise to much difference of opinion, though 

 most geologists of the present day are agreed that, however 

 modified by circumstances, they are ultimately to be attri- 

 buted to the application of some moving power. 



There are certainly many instances of curved strata 

 which might possibly have originated from mechanical vio- 

 lence, provided these strata were, when bent, in a plastic or 

 yielding state ; but what evidence have we of the existence 

 of such ductility in these rocks coeval with the application 

 of a moving power ? and, in examining this question, let us 

 confine our attention to the primary rocks, a restriction 

 which is most favourable to the supporters of this opinion. 



It is presumed that the advocates of the Plutonic theory 

 will not select the period of deposition, when gneiss and 

 mica-slate are supposed to have existed as unconsolidated 

 sediments, but would rather adopt the metamorphic stage, 

 whilst the primary schists were in a state of intense ignition 



