" Researches iti Physical Geology." 7 



rive at a just estimation of the individual value of the moving 

 power; but it would seem to be almost impracticable to cal- 

 culate the result of such an action on a mass traversed bv 

 joints at acufe angles to each other, — a condition found to be 

 very prevalent in existing rocks. I am aware that it is not 

 generally admitted that all solid rocks have a Jointed structure; 

 but Mr. Hopkins ought not to object to it, as he regards such 

 structure to have proceeded from an " internal molecular ac- 

 tion." 



Now, if rocks having a jointed structure be acted on by an 

 expansive fluid so as to overcome their continuity, it might be 

 expected, as I have elsewhere stated, that the dislocations 

 would take place on the lines of structure, as being the direc- 

 tions of least resistance. Mr. Hopkins also adds an excellent 

 practical remark, " that the accuracy of coincidence, between 

 the lines of structure and of fissure, is essential to the theory 

 which would assign the latter phsenomena to the prior exist- 

 ence of the former. A difference of a few degrees in the an- 

 gular position of the above lines would, if clearly established, 

 be fatal to this theory: because, as I have already explained, 

 although a fissure produced by an elevatory force would cross 

 a line of less resistance under a certain condition without 

 change of direction, that condition cannot be generally satis- 

 fied when the angle between the fissure and line of less resist- 

 ance is small ; in such a case the fissure will be propagated 

 exactly along the latter line. " Such an occurrence, it ap- 

 pears to me, might rather suggest a doubt concerning the 

 supposed action of an elevatory force, since our daily ex- 

 perience shows that all solid rocks do now possess a jointed 

 structure. Mr. Hopkins, however, is of opinion that should 

 this coincidence be established, it is more probable that the 

 lines of structure have been influenced by the dislocation of 

 the mass than vice versa. This result of the mathematical 

 analysis seems to have reduced the argument in favour of an 

 elevatory force ad absurdum ; for undisturbed horizontal strata 

 have a jointed structure, and in rocks having a rhomboidal 

 structure the joints meet at acute angles, which Mr. Hopkins 

 himself states must necessarily have been owing to some dif- 

 ferent cause than that of extraneous forces, such as extension 

 from elevation or contraction during solidification. 



Mr. Hopkins has visited Cornwall, and found it, like others, 

 a stumbling-block. He does not view its perplexing phae- 

 nomena as indicative of a defect in the theory of geology, but 

 regards them as exceptions to the general rule. " In this 

 mining district," he says, " it appears most necessary to re- 



