1808.] nOLL SOUTH DEVON AND EAST CORNWALL. 427 



occupy, for the most part, the low ground, and dip into the hills on 

 the east and south-east, which are composed of slates and ash beds 

 dip2)ing in the same direction, and rising steeply 200 feet or more 

 above the limestones. This is well seen at Pridhamsleigh, in the 

 descent to Ashburton from Goodrington, and in the hills north of 

 Bickington, where the limestone is overlain by volcanic rocks. 

 Even supposing, therefore, that the lamination does not rej)resent 

 the true bedding, it will still appear that the slates are uppermost. 



The relation of these slates to the overlying limestones will be 

 considered in connexion with the latter. 



2. Plymouth and Toi'hay Limestones. —-Th-Q Plymouth mass of 

 limestone commences on the west at Impacombe, south of Devon- 

 port, where it is overlain by the slates and red rocks of Mount 

 Edgcombe ; and slates, for the most part blue, pass under it on the 

 north ; but its relation to the rocks on the west is obscured by the 

 waters of the Hamoaze and JSango Lake. I am unable, therefore, 

 to bring this limestone into connexion with the contorted beds of 

 St. John's, among which, as already noticed, there are some calca- 

 reous slates ; but the higher beds of Impacombe appear to run out 

 seaward by Millbrook and Withnoe, above the purple and greenish 

 slates of Freathy, which include some bands of volcanic ash. It is 

 the same with its eastern termination, which appears to thin out 

 horizontally or nearly so. As observed by Sir Henry De la Beche, 

 however, there is much ambiguity about the bedding of this lime- 

 stone, although the general dip of the mass is to the south, so that 

 its relations and thickness are difficult to ascertain. The joints are 

 very regular, and in places where the rock is highly crystalline the 

 true bedding is very obscure. There can, however, be little ques- 

 tion that the limestone overlies the variegated slates of the north 

 of Plymouth, and dips under the grey and blue slaty rocks of Mount 

 Edgcombe, Plimstock, and Elberton ; and between its outcrop and 

 its dip under the higher rocks, it appears to form one or more undu- 

 lations, so that its real thickness may be very much less than its 

 superficial breadth and apparent dip would seem to indicate. 



In following the line between the slates which underlie and over- 

 lie the Plymouth limestone to the eastward, we are assisted by the 

 volcanic rocks of Hearston and Filham House, which lead up to the 

 fossiliferous slates of Black Hall on the Avon. No- limestone, how- 

 ever, occurs on this line of strike until we reach Sandwell, where 

 there is a small patch overlain by volcanic rocks ; and beyond this 

 we have the somewhat larger patch of Paytor, which is brought 

 down by a fault bounding it on the north ; and apparently the lime- 

 stone of Woolstone Green is also faulted down. There is a fourth 

 small patch at West Ogwell, which together with those just named 

 ajDpear to form the thin western margin of the range of limestone, 

 which becomes more largely developed immediately to the eastward 

 of them. The great mass south-west of Newton Bushell, which 

 constitutes the Ogwell and Ipplepen limestone, forms a tableland 

 of slightly undulating beds, denuded and excavated in the vicinity 

 of East Ogwell and in the valley of the Torbryan brook, so as to 



