Prof. Sedgwick on the Structure of large Mineral Masses. 485 



joints may have a given relation to the meridian of the place ; and at the 

 same time, such a given direction may be a mere secondary phaenomenon, 

 resulting from the original direction of the mountain chain. 



After the experiments of Mr. Fox, it becomes, however, a question of some 

 importance, whether the structure of large mineral masses may not have been 

 affected by the direction of great electrical currents at the period of solidifi- 

 cation : and if this be the case, may we not look for certain joints and 

 cleavages, preserving a parallelism through independent chains, and having 

 no fixed relation to the position of the masses they traverse.? Such a question 

 requires for its solution a series of observations on different mountain chains, 

 of which the directions are strongly contrasted. As far as my own experience 

 is concerned, I should answer it in the negative, as I believe that all joints 

 and cleavages originating in the structure of a rock are greatly modified by 

 its direction. 



In limestone rocks we sometimes see two sets of joints, inclined to each 

 other at angles very nearly equal to those of the primary rhomb of carbonate 

 of lime. Mr. Conybeare has pointed out to me some joints of this kind in the 

 lias. I have seen many examples of them in the mountain limestone of the 

 North of England ; and the late Dr. E. D. Clarke biought beautiful primary 

 rhombs of compact limestone from the mountains of Greece. It becomes, then, 

 a question, whether joints like these are mere local pliaenomena, or affect 

 large masses of strata ; and the inquiry ought to be kept in sight by those 

 who compare, on a large scale, the joints of calcareous mountains. At all 

 events, in examining the jointed structure of rocks, not only the directions of 

 the joints ought to be recorded from the compass, but the direction of the 

 mineral masses must also be recorded, otherwise the facts may lead us into 

 entirely false generalizations. 



Lastly : there may be in rocks, both stratified and unstratified, distinct sets 

 of parallel joints or fissures, extending through considerable tracts of country, 

 yet of mere mechanical origin, and unconnected with any peculiarity of in- 

 terna] structure. The subject has been lately discussed before the Cambridge 

 Philosophical Society, by Mr. Hopkins ; and lie has shown, on mathematical 

 principles, that tabular masses of rock elevated by a force from below, must 

 have been exposed to two sets of tensions, which would naturally produce 

 longitudinal and transverse vertical fractures, at right angles to each other. 

 Having solved the problem as a mere abstract question of mechanics, he then 

 showed that the direction of the metalliferous veins, and of certain longi- 

 tudinal faults in Derbyshire, coincided with the deductions of theory more 

 nearly than might have been expected. I may add, that an examination of 



VOL. III. SECOND SERIES. 3 R 



