476 Mr. Austen on the 



a cliff of some elevation. The intrusive nature of the dyke is proved by the great 

 changes it has produced along its course : the slates have lost their colour, and are 

 either porous or baked and splintery ; the limestone shales have been fused into 

 jasper ; the solid limestone has been crystallized to a very considerable distance 

 from the dyke ; the new red sandstone has been hardened into a compact mass, 

 and the lines of stratification are in many places quite obliterated ; but there are 

 innumerable vertical fissures, which are coated with manganese. This altered rock 

 is quarried above Bishopsteignton, as well as on the opposite side of the dyke, 

 above Lindridge ; and its course in the new red sandstone can be carried on by 

 several abandoned excavations. 



Certain appearances, such as that of the conglomerate above noticed, seem 

 to require for their solution the supposition, that this was a subaqueous erup- 

 tion ; whilst the porous vesicular nature of some of the trap would indicate that 

 the pressure of water was slight. With respect to the angular blocks contained in 

 the compact greenstone, we must suppose that the liquid mass brought up with it 

 from below, in the same manner as modern lavas, portions which had become so- 

 hdified. 



If the highly inclined position of the beds at Staple Hill, of which the upper 

 portion is certainly post-cretaceous, has resulted from the local elevation of the 

 slate on which they rest, and this elevation is owing to the great mass of trap 

 which the hill contains, we ascertain the age of another band of erupted trap ; and 

 there are appearances exposed in a road-section on the opposite side of the same 

 hill, on the way up to Ingsdon, which confirm this inference. 



The granite of Dartmoor presents a similar composition and texture over the greater part of its area. 

 The principal portion is porphyritic, containing large crystals of felspar, but its constituents vary locally 

 in their relative proportions. At one place, near Oakhampton, is a very beautiful white felspathic rock, 

 resembling pure trachyte : another form is that in which schorl takes the place of mica, as is the case 

 along the edges of the moor, but I know of no spot where the mica is developed in such large plates as it 

 usually is in most other extensive granitic masses. Wherever bare surfaces of this granite are ex- 

 posed, as along the upper courses of the Teign and Dart, linear veins will be observed, filled with 

 compact schorl. They are never very wide, and have the same direction (about east and west), which 

 is that of one set of joints, but I have never seen a cross set so filled. There is another set of veins, 

 usually much wider than the preceding, filled with matted crystals of quartz and schorl, which evidently 

 occupy cracks opened in the rock after it had become hard and compact ; and the growth of the contents 

 of the fissure has been by successive additions from the walls on each side, until the two portions met, 

 in the middle. Geodes also occur in the granite, lined vvith schorl, and crystals of quartz. Similar 

 cavities have afforded large and beautiful specimens of tourmaline. These minerals give a very distinct 

 character to a portion of the Dartmoor granite, which occurs universally along the edges of the mass 

 and at various places within the interior, often forming tors, surrounded by a rock of a very different 

 composition and appearance. 



The granite of Dartmoor is altogether an intrusive mass ; and it was stated by Prof. Sedgwick and 



