1868.] iioLL — so urn devon and east Cornwall. 407 



ravel the structure of this intricate country this distinction becomes 

 of importance, as these interbedded volcanic rocks afford great assist- 

 ance, inasmuch as they serve to separate the slates and grits into 

 horizons, and otherwise act as landmarks to the geologist. 



If, now, we endeavour to picture to ourselves a broad sheet of 

 lava, ash, and cinders spread out horizontally round about Brent Tor, 

 reaching from what is now the Tamer to the Tavy, and southward 

 to the parallel of Tavistock, and then imagine this sheet of volcanic 

 rocks, together with those that underlie it, thrown into a narrow 

 anticlinal fold along a line extending from Eamsdown past Dunterton 

 to the Tamer — and a second such narrow line of elevation passing 

 through Upperton towards Bowdon Down — and a third similar, but 

 longer and yet narrower axis extending from the south of Milton 

 Abbots and north of Lamerton towards Petertavy, at the same time 

 depressing the area about Heathfield Down so as to trough higher 

 beds, and raising that of Black Down on the east, we have a rough 

 idea of the arrangement of the beds before us. Thus the volcanic 

 rocks of Lamerton are a more southern portion of those of Milton 

 Abbots and Charlhanger, thrown over a long sharp anticlinal axis, — 

 narrow on the west, where they dip under the slates and grits of 

 Two well Down, and broader on the east of Lamerton, especially in the 

 vicinity of Kilworthy and Wilminston, where they lie more horizon- 

 tally. So with the volcanic beds of Upperton and Wick, which are a 

 part of the same as the Milton Abbots beds rising up again from be- 

 neath the slate, chert, and grit of Heathfield Down, to be thrown 

 over to the north at Quether. The ash extending from South 

 Brent Tor to Burn, parallel to the railway, remarkable for its highly 

 vesicular structure, appears to be the continuation of the Milton 

 Abbots ash-bed faulted off at Burnford Farm, and thrown over to 

 the westward of the anticlinal axis of Black Down ; and the ash-bed 

 of Bowdon belongs to the same geological horizon. The section 

 (fig. 1, p. 409), across Heathfield Down, from the MiU Hill Slate- 

 quarry, near Tavistock, to the Thistle Brook at Stowford, will show 

 the general relations of these rocks. 



Several faults appear to traverse the country in the vicinity of Brent 

 Tor. One of these crosses the Lyd river south-west of Coryton, and 

 ranges by Monkstone to the west of the Tor. Another skirts the north- 

 east side of the volcanic rocks of the Tor, extending from near Monk- 

 stone to South Brent Tor. These faults carry the country on the east 

 further to the south, or rather south-east, and reverse the dips along 

 the valley of the Tavy, where the beds rise to an anticlinal axis, 

 which crosses the railway in a N.N.E. and S.S.W. direction, midway 

 between Ford Gate and the Marytavy Railway Station. 



and hornblende. Augite and hornblende are of less frequent occurrence, and 

 then only in the more compact varieties. Many of these rocks are highly 

 calcareous, the lime being sometimes diffused among the volcanic materials as a 

 constituent of the rock ; in other cases it has been merely infiltrated into the 

 cavities of vesicles at a subsequent period. Near the granite these rocks have 

 been altered both in their crystalline condition and in the arrangement of their 

 elements ; and of the resultant minerals hypersthene appears to be one. See 

 further De la Beche, Rep. p. 119 et seq. 



