468 W. A. E. USSHER ON THE CHRONOLOGICAL VALUE OF THE 



is probable that similar beds may be concealed by the sands, so that 

 an approximate thickness cannot be arrived at. 



Northwards from the Crediton valley the lowest division is gene- 

 rally represented by sands with local intercalations of, and possibly 

 replacement in their upper beds by, clay, on the low grounds ; 

 whilst the Palaeozoic boundary is flanked by Triassic gravels of local 

 materials resting on considerable heights (exceeding 900 feet above 

 the sea at Cadbury Camp, north of Thorverton) on the- west side of 

 the Culm valley, between Silverton and Tiverton, and well developed 

 on the flanks and near the western extremity of the Tiverton valley. 

 This gravel appears to have originally exhibited the same relations 

 to the sands and clays of the division, that exists between the Dolo- 

 mitic conglomerates and marls of the Men dip area ; for it is never 

 visible at considerable distances from the exposed palaeozoic rocks. 



As this Triassic gravel rests on slopes of some hundred feet in 

 vertical extent, we may regard the basement-beds whose shoreward 

 margin it represents as about 600 feet in thickness. 



In the Tiverton valley the sands of the lowest division seem to 

 give place to breccio-conglomerate near Sampford Peverell ; this ap- 

 proaches in character, sometimes to the hard beds of the south- coast 

 breccia, sometimes to the Keuper conglomerates of Thorn St. Mar- 

 garet's and Wiveliscombe. On following this division northwards, 

 we find a considerable attenuation, real or apparent, between Samp- 

 ford Peverell and Wiveliscombe : toward the latter place the top of 

 the division consists of breccio-conglomerate, seldom exceeding 50 

 feet in thickness, the lower beds being represented by sands. 



In the Watchet valley breccia forms by far the larger portion of 

 the whole division, and the sands are intercalated with it. 



In the neighbourhood of Stogumber the basement-beds may ex- 

 ceed 500 feet. 



(Ji) I am inclined to think that this division is not represented by 

 contemporaneous deposits in the Bridgewater area ; if so, they are 

 of a very different character : nor can I say if the Lower Marls are 

 represented by equivalent deposits on the north-east of the Quan- 

 tocks. 



(i) It would be worse than idle to attempt a correlation of the 

 variable basement-beds of the Devon and West-Somerset Trias with 

 the Bunter divisions of the midland counties. There are occasional 

 points worthy of comparison between them, as the degeneracy of the 

 Bunter pebble-beds into a loose gravel in places *; but even if coin- 

 cidences were marked and numerous, the different conditions expe- 

 rienced by each area would preclude the derivation of reliable results 

 from them. 



(j) Taking the division as a whole, however, we may compare the 

 following comparative thickness of the Bunter in the midland coun- 

 ties with our lowest division with advantage. 



Thickness of Bunter. — In Lancashire and W. Cheshire, 1200 to 

 1750 feet ; in Staffordshire, 150 to 600 feet ; in Leicestershire and 



* Mem. Geol. Sur. on Triassic &c, of Midland Counties, p. 59. 





