686 Professor Sedgwick and R. 1. Muuchisonv, Esq., on the 



The leading system of joints is the same with that observed in Cornwall, 

 whether we examine the granite or the altered rocks that are close to it. 

 This remark applies to the culm measures, as well as to the older slates; and 

 the fact seems to show, that the joints (common to the granite and contiguous 

 slates) must be contemporaneous. But it by no means follows from this con- 

 clusion, that we should thence infer, as has been done by Dr. Boase, that the 

 granite and the contiguous slate rocks are also contemporaneous. Such a 

 conclusion is manifestly contradicted by the facts stated in the preceding 

 parts of our paper. 



3rd. Granite veins, passing from the central mass into the superimposed 

 stratified rocks, are found on all sides of Dartmoor. We liave seen them above 

 Ivy Bridge, injected among the oldest slates of Devonshire ; and near Oak- 

 hanipton, we have seen them in like manner, penetrating the culm measures : 

 and they are finely exposed in the beautiful gorges of the Teign and the 

 Dart, where those rivers descend from the granite to the culmiferous series. 

 IMiese examples, to which we could add many more, are sufficient for our 

 purpose. Now these veins, taken in general, are mere prolongations of the 

 central granite, inseparable from it, and contemporaneous with it ; they 

 cannot therefore (as Ihe granite is one mass) be contemporaneous with stra- 

 tified rocks of different ages. Consequently they are true veins of injection, 

 and the granite was protruded at a time, posterior to all the other stratified 

 systems. 



Again, the beds and masses of trap, associated with the stratified rocks, do 

 not penetrate the granite (as they do in the Isle of Arran and many other 

 places) but are uniformly cut off by it. This seems to prove that the injected 

 trappean masses obtained their present position among the slates before the 

 granite existed in its present form ; and the same conclusion is still more 

 certain when applied to the stratified, or recomposed, trappean rocks which 

 alternate with the slates. 



4th. The strata round Dartmoor are not all mantle-shaped. On the north- 

 western and north-eastern sides of it, their strike is not much altered by the 

 granite; and the successive beds may be said to abut against, rather than 



south joints about five degrees east of magnetic north. Mr. De la Beche, in his recently pub- 

 lished /?e2)or<, gives many accurate details connected witli this subject, which on the whole confirm 

 the statements first given by Mr. Enys and Mr. Fox. A person after reading these statements 

 might perhaps be disappointed on commencing an examination of the granitic hills of Cornwall, 

 and finding the joints often quite anomalous in their directions. For reasons already stated, such 

 an examination must not begin on the outskirts of the granitic hills, and no mean directions can 

 possibly be established, except on a large number of averages. (1839.) 



