Mr. Hopkins's " Researches in Physical Geology'^ 17 



we must allow the melted granite to cool, and the supposed 

 belt of altered slate to assume its new or superinduced lines 

 of structure. This being accomplished, then follows the — 



2nd. Movement, as denoted by the formation of fissures, in 

 which the dykes of elvan now occur, and which run con- 

 tinuously through both granite and slate. We have seen 

 that the latter rock was already solid, and there can be little 

 doubt that the granite was also in a similar state ; for very 

 highly inclined fissures, twenty to fifty feet in width, could 

 scarcely have been maintained in an ignited mass, possessed 

 of any degree of viscidity ; because, if in any state short of 

 solidity, the incumbent pressure would cause the mobile 

 mass to sink into and obliterate such fissures. 



3. The movement which gave origin to the metalliferous 

 veins seems also to have operated on solid rocks, since they 

 cut through granite, slate, and elvan which must have been 

 previously in a state to furnish considerable quantities of an- 

 gular portions of all these rocks with which the veins abound : 

 independently of the fact that this movement was subsequent 

 to former dislocations which, it has been shown, occurred in 

 rocks already solidified. 



4. The movement, indicated by the cross-courses also 

 containing detached portions of rocks, must likewise have 

 been effected in a solid mass. It is superfluous to make any 

 further remark on this head ; but I will here observe, that 

 Mr. Hopkins has fallen into an error in stating that cross- 

 courses are universally recognised to be of irregular width, as 

 compared with the " bearing " veins. The fact is, that both 

 systems are exceedingly irregular in this respect ; but if any 

 rule obtains, it is the reverse of Mr. Hopkins's statement. 

 Some importance appears to have been attached to this differ- 

 ence in width, but I cannot detect the nature of its bearing- 



From these facts* it appears, that in Cornwall the rocks 



* In discussing this subject I have endeavoured to keep the argument 

 as simple as possible, and therefore have not dwelt on the phaenomena of 

 veins. But as a specimen how the difficulty of Mr. Hopkins's position in- 

 creases when applied to some only of the details, I may observe that two 

 parallel systemsof veins frequently occur inclined towards, and intersecting 

 each other, at great angles ; whilst they aie both traversed by a third 

 system or cross-courses. All these veins are parallel to, and partially 

 identical with three systems of joints or lines of structure, dividing the mass 

 into concretions which are generally of a rhomboidal form. Here, then, 

 (without complicating the matter still further with joints and veins, which 

 in Cornwall, and probably in other countries, traverse the quadrilateral 

 concretions diagonally,) we have systems of veins crossing each other at 

 acute angles, a condition which Mr. Hopkins has stated cannot have been 

 produced by the elevatory or other extraneous forces, as these, he says, 

 "could only tend to produce systems of fissures crossing each other at 

 right angles." 

 ' Third Series. Vol. 10. No. 58. Jaji. 1837. D 



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