302 VOLCANO OF BRENT TOR. 



Volcano of Brent Tor. Mr. Allport suggested that Brent Tor, 

 which is four miles west of Tavistock, presents many of the features 

 of a volcano. The rocks consist of purple-bedded ash, with vesicular 

 and amygdaloidal dolerites, described by Mr. Rutley 1 as having 

 a fissile texture, and abounding in small crystals. They sweep 

 round Brent Tor in a semicircle, dipping from it at a low 

 angle ; and it is suggested that the old volcano has been faulted 

 through the cone, so that the ashy beds, by being thrown down, are 

 much better preserved on one side than on the other. The altered 

 dolerite about Tavistock is probably only a prolongation of the lava- 

 flows from Brent Tor. 



On the east of Dartmoor there are several dolerites of similar 

 character. Those seen near Hennock, to the N.E. of Bovey Tracy, 

 have the augite but slightly altered. Near Torquay eruptive dolerites 

 are exposed in Babbicombe Bay and in Austis Cove. These dolerites 

 are converted into a serpentinous rock at Clicker Tor, S.E. of 

 Liskeard, but more frequently show the alteration due to the develop- 

 ment of hornblende. 2 



Volcanic Rocks of the Mendip Hills. At Down Head Com- 

 mon, near Shepton Mallet, is a large exposure of intrusive rock, which 

 was regarded by Mr. Charles Moore as a dyke. It occurs in the Old 

 Red Sandstone. At Down Head the rock appears to be a felstone of 

 dark-grey colour, with minute crystals of hornblende and much 

 magnatite. At Stoke Lane in the Mendips, the lava has the cha- 

 racters of a pitchstone porphyry. It is a brownish-grey rock, with 

 minute vitreous crystals, which are colourless or greenish. Under the 

 microscope it is seen to consist of orthoclase and plagioclase, with a 

 little magnatite and a green mineral termed viridite. Mr. Rutley 

 has also described basalts and dolerites from the Uphill Cutting, 

 Great Western Railway ; Wrigton Warren, near Bristol ; Wood Spring 

 Hill, Charfield Green, and Damory. Some of these lavas are 

 amygdaloidal, and some are greatly decomposed. 3 



Felsite of Bittadon. This is an intrusive mass in the grey 

 unfossiliferous slates which form the upper part of the middle 

 Devonian rocks, and like the slates is affected by cleavage, so that 

 Professor Bonney dates its intrusion before the close of the Carboni- 

 ferous period. It was originally, he says, probably a sanidine 

 trachyte, with hardly enough quartz to be a rhyolite, and may have 

 been not unlike some of the Drachenfels trachyte ; but the original 

 minerals have undergone much alteration. The mass of the rock is 

 greenish-grey, thickly studded with small reddish-white crystals, 

 which are mostly orthoclase. A few small grains of quartz are 

 visible. Under the microscope it is crowded with indistinct micro- 

 liths, brown and green granules, and occasional black specks and 



1 F. Rutley : "On Schistose Volcanic Rocks on the West of Dartmoor," Q. J. 

 G. S., vol. xxxvi. p. 285. 



2 Allport : " Metamorphic Rocks Surrounding the Land's End," Q. J. G. S., 

 vol. xxxii. p. 418. 



3 Q. .T. G. S., vol. xxiii. p. 452 ; and Mem. Geol. Survey: East Somerset 

 and Bristol Coalfields, H. B. Woodward, pp. 14-208. 





