376 W, A. E. USSHER ON THE TEIASSIC 



at Yawford they are concealed by marl, but at once emerge, and 

 thence continue along the southern flanks of the Quantocks at 

 Bishops Lydeard and in the Watchet valley, finally becoming por- 

 tions of the Upper Sandstone division of the Devon and West- 

 Somerset Trias, hereafter to be treated of. 



Thickness. — Considerable difficulties exist in estimating the thick- 

 ness of the Trias in this area. Mr. Moore (" Abnormal Secondary 

 Deposits," 7. c . p. 458) mentions a boring made at Compton Dundon, 

 south of Street, through a small thickness of Khoetic beds, in which 

 the base of the Keuper was not reached at a depth of 609 feet. 

 If we allow a persistent clip of 3° in the Bridgewater Trias, measur- 

 ing its outcropping breadth from the Ehcetic beds on the Polden 

 Hills, near Puriton mill, to the Quantocks near north Petherton, 

 we get a thickness of about 1200 feet ; but the conditions for 

 forming such an estimate are too unfavourable to allow of its being 

 put forward except as a theoretical possibility. In the Yale of 

 Taunton, De la Beche allowed a thickness of nearly 300 feet for 

 the Trias ; but it is plain that the materials for forming an estimate 

 aro equally wanting there ; for the vicinity of the district to the 

 Watchet and Wellington areas, where a very much greater thick- 

 ness and more variable composition are exhibited, and the uncertainty 

 as to the depth of the old rock floor under Taunton render it pos- 

 sible that the marls, -superficially constituting the Trias, may con- 

 ceal beds of a different character, of the same age as the conglomer- 

 ates or lower marls of the Watchet and South-Devon area. 



We can only partially account for so small an estimate of the 

 thickness of the Yalc-of-Taunton Trias by assuming that the rocky 

 floor on which it rests formed, as a ridge extending southwards 

 from the Quantocks, a barrier to the incursion of Triassic waters 

 during the deposition of the breccias, lower marls, and conglome- 

 rates in the area between Watchet and the south coast of Devon, 

 and during the earlier deposition of sandstones in the Bridgewater 

 district, not finally coming within their influence till the later 

 stages in the deposition of the Upper Sandstones of West Somerset 

 and Devon. Even on this assumption, allowing a persistent dip of 

 3°, and estimating breadth of outcrops from the base of the Bhaetic 

 beds north of Stoke St. Mary to the Quantocks at West Monkton, 

 the resultant 700 feet can scarcely be considered excessive. 



Concluding remarks. 



We have hitherto been considering districts in which the Trias of 

 Somerset presents its simplest form, being mainly divisible into two 

 portions, the lowermost of which appears to be far the least developed. 



In the Mendip area we find : — (1) Yariegated marls occasionally 

 arenaceous or passing into sandstones at and near their base, over- 

 lying or dovetailing into (2) Dolomitic conglomerate, which though 

 apparently the first deposit of Triassic times in the area, has, in 

 places, as a marginal deposit, continued during the entire deposition 

 of the marls. 



In the Bridgewater district, and in the area between Taunton and the 

 Polden Hills (as far as we can judge by superficial evidence) we find^: — 



