410 I'EOCEEDTXGS OF THE GEOLOGICAL SOCIETY. [April 22, 



the south, beyond which are the southernmost limestones of Tim- 

 ber Bridge, near Lifton, and of Tinhay, and those of Lew Tren chard, 

 Point Bridge, and Bridestow, which return by Thrastleton, Stowford, 

 Thistle Brook, and the smaller patches north of Tinhay and Timber 

 Bridge, the included area being occupied by the lower rocks of the 

 Lyd river brought up to the surface by the anticlinal axis of Old 

 Street Down. 



The chert of Gordon Hill is not continued to the west on the 

 same parallel ; but on St. Stephen's Down, on the road from Laun- 

 ceston to Yeolm Bridge, similar chert occurs, and ranges westward 

 towards Egloskerry. This chert occupies the centre of a synclinal 

 trough, the underlying Devonian rocks being brought up to the sur- 

 face along a narrow axis on the north, which extends from Under- 

 wood Parm to Yeolm Bridge, while a lower axis on the south ranges 

 east and west through the town of Launceston. A bed of limestone, 

 together with some volcanic ash, underhes this chert at Calvanna 

 Park, near Lower Truscott, which bears the same relation to the 

 siliceous rock above as the limestones of Coryton and Timber Bridge 

 do to the chert of Gordon Hill and Sydenham. Between the anti- 

 clinal axis of Yeolm Bridge and the lower rocks of South Petherwin 

 we appear to have, therefore, the whole of the Culm-measure series 

 from the limestones downwards ; but we miss from the rocks south 

 of Launceston much of the volcanic ash, and most of the grit and 

 chert which characterize the Carbonaceous system along the banks 

 of the Tamer. Brent Tor, or its vicinity, as already suggested by 

 Sir Henry De la Beche*, was probably the centre of volcanic action, 

 and this may sufficiently explain the somewhat local distribution of 

 the igneous products. The rocks forming the Tor are composed 

 in great part of cinders, some of large size, mingled with ash, 

 cemented into a kind of conglomerate, the vesicles being filled some- 

 times with carbonate of lime, at others vrith chalcedony. J^either 

 in their mineral nor in their physical character do these volcanic rocks 

 differ more than those of modern eraptions ; the lavas are some- 

 times vesicular, at others crystalline and compact, the ashes more 

 or less schistose and mixed with cinders, and the whole commingled 

 in a manner that shows their common origint» 



It is difficult, therefore, where igneous and arenaceous rocks are 

 intercalated so irregularly among the slates, to form even an approxi- 

 mate estimate of their thickness. If we take the distance from the 

 lower rocks brought up by the anticlinal axis at Underwood Parm to 

 the middle of the chert troughed in the synclinal fold on the south, at 

 Upper Truscott, the distancejis barely three-fourths of a mile, which, 

 at a maximum average dip of 35°, would give little more than 2000 

 feet ; and if to this we add half as much again for the increase of 

 arenaceous materials and intercalated volcanic rocks on the banks 

 of the Tamer, we shall probably arrive at as near an approximation 

 as we can make. 



* Rep. p. 122. 



t The vesicular structure of some of these rocks would lead us to infer 

 that they were accumulated, in part at any rate, on dry land. 



