of the Structure of Rocks. 45 3 



suggestions without alluding to their having been already pro- 

 pounded. 



Thus, at page 4*83, he observes that " if the grain of gra- 

 nite 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 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. Austle granite is nearly 

 parallel to the direction of the Cornish slates." 



Now in a paper read before the [Royal Cornwall Geologi- 

 cal] Society in the years 1830 and 183 J, and published in the 

 Transactions in 1832, I stated that the masses of granite di- 

 vided into layers by seams or joints, are sometimes perpen- 

 dicular, but more frequently inclined at various angles from 

 45° to 80° ; and that they only differ from the layers or strata 

 of the adjoining slate, in the one being fissile and the other 

 not ; in short, that they both possess the same concretionary 

 structure, the result probably of a peculiar crystalline ar- 

 rangement of their constituent minerals ; — and lastly, that beds 

 of granitic rocks, forming integrant parts of the central masses, 

 sometimes extend beyond the boundaries thereof, and alter- 

 nate with the slate. Again, in my " Treatise on Primary 

 Geology," I have often dwelt on this topic, showing that the 

 layers of granite vary in composition and alternate together 

 precisely after the same manner as the crystalline slates ; that 

 they have often determinate directions, as recorded by several 

 observers in different countries ; that even when the joints are 

 not visible they may be cleft most easily in directions parallel 

 therewith, as pointed out by Mr. Enys; and lastly, which is 

 to the point, that " the direction or strike of these granitic beds 

 is placed in a certain and determinate position, which in Corn- 

 wall is parallel to the most frequent course of the adjacent schis- 

 tose rocks." For further details on this subject the 6th, 10th, 

 12th, and 16th chapters may be more particularly consulted. 



The Professor next adds : "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 direc- 

 tion of the nearest metalliferous veins. As so many rocks 

 are intersected by cross joints, nearly perpendicular to their 

 strike, we might expect, d 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 ■ master joints' or 'cross 

 courses' in that country nearly at right angles to the bearings 

 of the central chain." 



