1868.] nOLL SOUTH DEVON and east CORNWALL. 405 



dip, arc also seen in the river above the bridge, and in a slate-quarry 

 on the opposite side of the river. But a little further up the stream, 

 opposite a place called Bridge Farm on the map, horizontal beds of 

 dark-grey micaceous sandstones vrith carbonaceous slates occur, oc- 

 cupying lower ground than the roofing-slates in the lane ascending 

 from the bridge to the quarry. These Carbonaceous rocks set in 

 immediately beyond the copper-lode which has thrown them down on 

 the north. The grits contain plant-remains, and resemble very closely 

 those exposed at Lower Down House, west of Beal's Mill, and in the 

 quarry north-east of Nighton. This copper-lode forms the southern 

 limit of the Culm -measures all the way to Hartwell, beyond which 

 the line of their outcrop curves northward with the high ground to 

 Chipshop and Ottery. Westward of the latter place Carbonaceous 

 slate with chert is seen in nearly horizontal position, although much 

 contorted, while close by, in the adjoining field, the grey roofing- 

 slates of Mill Hill quarry dip E. 40° N. at high angles. From the 

 northern side of this quarry the Culm-measures are continued 

 to Stiles Wick and Downhouse Farm, and thence, thrown down 

 by a fault, they cross the Tavy at the southernmost of the three 

 bridges at Tavistock to Challicot; but they are difficult to follow across 

 Whitechurch Down from want of exposures and, as we approach 

 the moor, from the alteration in the mineral character of the 

 rock produced by the granite*. The Culm-measure slates with 

 their thin grits are well seen in the bed of the river between the 

 bridges at Ta\istock when the water is low ; and chert is quarried 

 near the town and in the vicinity of Collytown, on the road to Mer- 

 riville Bridge. These rocks are exposed also in a small section at the 

 south end of the raihvay- station ; and the black slates have been 

 cut into in lowering the roads on the west of the town. A little 

 north of the railway -station thick-bedded grits dipping north-west 

 are faulted against a bed of highly calcareous volcanic ash dipping 

 in the opposite direction. Westward of this is an anticlinal axis fol- 

 lowed by a synclinal trough, which runs in a north-easterly direc- 

 tion by Tavytown, south of which the Culm-measures crop out on 

 Whitechurch Down. The volcanic rocks, as they rise to the south 

 on Whitechurch Down, consist chiefly of what appears to be com- 

 pact chlorite with grains of quartz, and dip N.N.W. at an angle 

 of 30°. The town of Tavistock lies in a synclinal trough running 

 north-east and south-west. The rocks are much disturbed and con- 

 torted, and there are many small faults in the vicinity of the town. 



At Pentre Cross, near St. Mullion, several miles to the south, 

 there are two small outlying patches of these Culm-measures brought 

 down by an east and west fault, with a downthrow on the north, 

 which crosses the turnpike road a little south of the turn-off to 

 Callington. There is no mistaking the character of these rocks, 

 which consist of black carbonaceous slate with chert, and massive 



* It may be possible that some of the altered rocks which skirt the granite 

 and form the high ground south of Stamford Spiney, and east of Walkingham 

 and Meavy, may belong to the Culm -measures, having escaped removal by de- 

 nudation. 



