474 Mr. Austen on the 



canic activity. The contemporaneous trap of tlie carbonaceous period is however 

 more local in its character, as it seems not only to be entirely wanting throughout 

 the central portion of the deposit, but also along the northern boundary, among 

 beds which in age we must consider to be the equivalents of the southern ones. 

 These subordinate trappean beds may therefore be briefly described as ranging 

 parallel with the southern outline of the carbonaceous series, and as partaking 

 along the north side of Dartmoor of all the flexures and dislocations which the 

 intrusion of the granite has produced. Good natural sections are exhibited along 

 the course of the Teign, where that river cuts the beds at right angles to the 

 strike ; at Crocombe, near Chudleigh, fifteen trappean beds, varying from a few 

 inches to several feet in thickness, alternate with black shales, and sections exhi- 

 biting similar phsenomena may be obtained in many other places. 



§ 2. Intrusive. — Though trap rocks have been erupted among beds of the car- 

 bonaceous series, the area occupied by the latter does not present, when compared 

 with an equal portion of the older slate system, the same number of instances of 

 intrusive masses. Beneath Ramshorn Down, 2^ miles N.W. of Newton, are pro- 

 trusions of greenstone, and the shales and flagstone are much altered ; at Penwood, 

 a little further east, the whole of a conical hill has been converted into flinty jasper ; 

 and at Hennock, 1^ mile N.W. of Chudleigh, as well as Botter, great masses of 

 erupted trap are in contact with, but do not enter the granite. 



The igneous products hitherto mentioned seem to have been exclusively horn- 

 blendic, but the close of the carbonaceous period was marked by eruptions of a 

 peculiar character, which produced the crystalline rocks of the neighbourhood of 

 Exeter. This trap has been so often noticed, that no details respecting its mine- 

 ralogical character are here requisite ; it is simply a porphyry, generally red, only 

 occasionally green, the base varying in texture from earthy to compact, and con- 

 taining large crystals of that variety of felspar which has been made a distinct 

 species, under the name of Ryakolite : near Silverton and some other places it con- 

 tains a large proportion of mica, and some hand-specimens can hardly be distin- 

 guished from dark granite. This rock is seen at the eastern extremity of Great Haldon, 

 at Dunchideock Bridge, &c., resting on the carbonaceous shales. It forms a consi- 

 derable portion of Pocombe Hill, in which are several quarries ; and on the western 

 slope of the hill, the road exposes a good section showing the position of the por- 

 phyry resting on the smooth surface of the black shales ; dipping as well as at 

 the same angle with them : beneath the castle of Exeter, it rests on shales, from 

 which place it plunges under the new red sandstone in the direction of Heavitree. 



It has long been a favourite notion, that the loM'est conglomerate beds of the new red series originated 

 in a disturbed condition of things, and that in some instances, as in the case of the porphyritic rocks of 

 Exeter, the true cause of the disturbance was apparent ; it will be as well therefore to notice this rock 



