Prof. Sedgwick on the Stnictiire of large Mineral Masses. 483 



nearly magnetic east and west, and they are never inclined to the horizon at 

 less than 70° or 80°. On the south side of this moor they generally underlie 

 to the south ; but in other places they are very nearly perpendicular, and at 

 Bunney '"^ open-work" they underlie north. The parallel stripes of schorl 

 rock often part in the middle, along- very narrow joints traced by oxide of tin. 

 Tin is also disseminated in regular crystals, both in the granite and schorl 

 rock, where we have no appearance of any regular vein distinguished from 

 the rest of the formation. In short, the whole rock has a laminated or veined 

 structure, produced by a peculiar segregation of parts, in passing from a state 

 of fusion into a solid state : and the parallel laminations must have been pro- 

 duced by a crystalline action, very similar, at least, to that which has so com- 

 pletely modified the ultimate structure of our slate rocks*. 



The grain of certain masses of granite, where we have no external appear- 

 ance of laminations, ma^, perhaps, have been produced by a similar crystalline 

 action. I have stated above, that the strike of the cleavage of lai-ge masses of 

 slate, nearly coincided with the strike of the beds. A mass of granite has (pro- 

 perly speaking) no strike; but it is often protruded upon a given line of direc- 

 tion. If, then, the grain of granite rocks be produced by a modification of 

 crystalline action, similar to that which produced slaty cleavage, have we not 

 some reason to expect that the grain of such rocks may be traced nearly along 

 their lines of protrusion ? I throw this out as a mere conjecture, suggested 

 by an analogy, and by the fact, that the veined structure of the St. Austell 

 granite is nearly parallel to the direction of the Cornish chain. 



The grain of a rock must evidently have considerable influence in modi- 

 fying the direction of fissures subsequently produced by mechanical force. 

 Hence the direction of the veins of fissure 7naj/ in some cases be considerably 

 affected by the previous direction of the veins of segregation. It would be 

 well, in a place like Cornwall, to institute a set of direct observations, for the 

 purpose of comparing the grain of the granite with the direction of the 

 nearest metalliferous veins. 



As so many rocks are intersected by cross joints, nearly perpendicular to 

 their strike, we might expect, ii priori, to find many great " master joints,' 

 nearly at right angles to the direction of the granitic ridge of Cornwall. At 

 all events, whether such reasoning be good or bad, there are many great 



* Such rocks as those above described must be carefully distinguished from true gneiss ; for 

 the parallel, vertical layers are as distinct from true strata as cleavage planes are from beds of 

 slate. A great part of the gneiss of Scotland is undoubtedly stratified ; but on the shores of the 

 Pentland Firth there are rocks, commonly described as gneiss, which, if I mistake not, are mere va- 

 rieties of veined granite, and are formed in the same manner as the granites of St. Austell Moor. 



