Physical Structure and older stratified Deposits of Devonshire. 685 



Chap. V. — Granite of Dartmoor ; its Structure and Relations to the Stratified 



Rocks, (Sfc. 

 It only remains for us to describe in a few words, the Dartmoor granite 

 and the phenomena presented at its junction with the other stratified forma- 

 tions : — 



1st. It presents the large porphyritic structure, the contemporaneous cry- 

 stalhne portions, and veins of schorl ; the portions liable to decomposition from 

 the disappearance of the alkaline constituent of the felspar ; the hemitrope 

 crystals of albite ; and in short, almost all the other well-known accidents of 

 structure, which have so often been remarked in the Cornish granite. In- 

 deed, there can be n6 doubt, that, from Dartmoor to Land's End, all the larger 

 protuberances of granite belong to one common system of formation ; and 

 they present the like phenomena at their junction with the stratified rocks. 



2nd. There are other accidents of structure, common to the granite of De- 

 vonshire and Cornwall, which we may also notice — more especially the joints. 

 Every one who had written on the granite of these counties, had noticed the 

 occasional prismatic, or cuboidal, forms of the rock; and Dr. McCuUoch en- 

 deavoured to account for these phenomena on the hypothesis of interfering 

 spheroidal concretions. Since that time, Mr, Fox and Mr. Enys have shown, 

 that two leading sets of these joints are nearly perpendicular to the horizon ; 

 and have (in the western parts of Cornwall) given bearings with respect to the 

 points of the compass, — one set being nearly magnetic N. and S., and the 

 other nearly magnetic E. and W. ; while a third set are nearly horizontal*. 

 Nor did their remarks end here ; for they showed, by the peculiarities of its 

 cleavage, and the prevalent arrangement of the larger felspar crystals, that 

 these joints were due to a crystalline arrangement affecting the whole mass 

 of the granite. These arrangements, however, fail on the outskirts of the 

 granite ; more especially in the deviation of the tabular joints, which, in such 

 cases, often assume a kind of dome-like shape, conforming rudely to the 

 actual surface of the country. Dr. Boase has also dwelt largely on these 



subjects, and shown that many of the accidents of structure, especially many 



master-joints, are common to the granite and the rocks in contact with itf. 

 Now all these important peculiarities of structure (first observed in the 



western parts of Cornwall) are equally true of the granite and contiguous 



rocks of DartmoorJ. 



* See Phil. Mag. May 1833. 



t See various Memoirs by Dr. Boase in the Transactions of the Royal Geological Society of 

 Cornwall. 



X From the mean of our observations we should place the bearing of the principal north and 



4t2 



