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



" master joints" or "cross courses" in that country, nearly at rii^ht angles to 

 the bearing of the central chain. 



1 have assumed above, that the veined structure of the St. Austell granite 

 was produced by segregation ; but it deserves remark that the several crystals 

 composing each part are by no means universally parallel to the great parallel 

 system of joints, though, on the whole, they rather affect that direction. Mr. 

 De la Beche has well pointed out, that tabular crystals of felspar will some- 

 times traverse such joints of granite*, so that a part of the same crystal will 

 be on one side of a joint, and a part on the other. He also describes an ap- 

 pearance on the north side of Dartmoor, of which there is a very perfect 

 example in the great open-work of Carglaze, near St. Austell. The alterna- 

 tions of schorl rock and granite above described become more frequent as 

 we approach the junction of the slate, and at last are so frequent and fine- 

 grained, that the rock, on the south side of the open-work, becomes finely 

 laminated, and passes into a true schist. This I noticed many years since, 

 in a paper published in the Cambridge Transactions f. What ought we to 

 infer from a phsenomenon like this ? That the slate rock in contact with the 

 granite had at one time been nearly in the same condition as the granite ; 

 and that both had been modified by a similar crystalline action in passing 

 into a solid state. Now all phajnomena of this kind accord perfectly with the 

 igneous theory of granite, and its protrusion among the stratified slates : yet 

 have they been urged as proofs that the slate rocks of Cornwall (including 

 in the list the fossiliferous slates of Tintagel, the coarse greywacke of the 

 south-eastern coast, the shales, the limestones^ the beds of greenstone and 

 felspar, &c. &c.) are all contemporaneous with the central granite. 



Metalliferous veins and cross courses, master joints and fissures, and other 

 similar interruptions to the continuity of great mineral masses, appear in most 

 instances to have originated in mechanical action. The observations applied 

 to certain formations of granite may, however, be applied to rocks of every age. 

 They may, from their mode of aggregation, or grain, have a tendency to break 

 in one direction rather than another; and hence one set of joints, though pro- 

 duced by mere rude mechanical action, may still have their directions defined 

 by an internal structure resulting from an entirely independent cause. Thus, 

 in many large formations of semi-crystalline limestone (such, for example, as 

 the great scar limestone of Yorkshire, or the limestone of the High Peak of 

 Derbyshire), there may probably be a grain bearing a given relation to the 

 direction of the whole mass. U such be the case, the direction of one set of 



* Researches in Theoretical Geology, p. 104. | Vol. i. p. 108. 



