Physical Structure and older stratified Deposits of Devonshire. 649 



Such are the groups mto which we have divided the deposits in the northern 

 region of Devonshire. They are founded on analogies of mineral structure, on 

 superposition, and on the existence of successive calcareous and fossiliferous 

 bands; and it is important (at least till they are better known) to keep them 

 distinct from one another. Some of the lines we have drawn, are, perhaps, 

 artificial and arbitrary ; but so are some of those which divide our best 

 known, secondary formations. Without something like the arrangement we 

 have adopted, it would be difficult to give any clear description of the great 

 deposits of the northern region ; still less could we hope to bring them into 

 comparison with the successive groups in South Devon. At all events, we 

 have shown that all the groups above described are inferior to the culm-mea- 

 sures of our third region, a fact with which we were unacquainted, when we 

 commenced our task. 



Chap. III. — Succession of Deposits between Dartmoor and the South Coast 

 of Devon, ^c. ^c. 



(See Map, PI. L., and Sections, PI. LI.) 



If a line be drawn (in a direction about east and west) from the limestone 

 cliffs near Plymouth to the limestone cliffs of Berry Head, at the south end of 

 Tor Bay, it will cut off the great south promontory of Devon, which terminates 

 in three remarkable headlands, — Bolt Head, Prawle Point, and Start Point. 

 Now, as in all this promontory the strike of the beds is nearly east and west, and 

 the prevailing dip is south, it appeared to us that the coast-section from Ply- 

 mouth to Bolt Head, and a corresponding section from Berry Head to Start 

 Point, must probably cross the same succession of deposits. We examined 

 both these sections, and our expectations were realized. North of the line 

 above mentioned, the strike and dip of the strata are by no means regular. 

 They gradually become affected by the granite of Dartmoor, and on dou- 

 bling its southern end they are bent towards the N.E. In this way, rudely 

 conforming to the mean boundary of the granite, they sweep round the 

 south-eastern side of Dartmoor with a north-eastern strike and a prevailing 

 south-eastern dip, as far as the basin of Bovey Tracey. If, however, we 

 make a traverse from this side of the granite to the cliffs of Tor Bay, we 

 gradually lose this regularity of strike and dip. The strata have been much 

 disturbed at different periods, and exhibit many axes of elevation (probably de- 

 rived from great protruding masses of trap) which conform to no given direc- 

 tion. The contorted beds of Tor Bay (which have been described in memoirs 



