370 W. A. E. USSHER ON THE TRIASSIC 



relatively to the sea, we do not suggest a mode of explanation at 

 variance with the manner in which such things may happen at the 

 present time, but on the contrary, one in accordance with it." 



He says further (ibid. p. 246), " The conglomerate of the Yale of 

 Westbury is pointed out as containing an abundance of sulphate of 

 strontia, forming part of the matter cementing the pebbles and frag- 

 ments together. The frequent occurrence also of nests of sand in 

 many places, the irregular cavities filled with rock crystal, chal- 

 cedony, carbonate of lime and other minerals, and which, when 

 detached from the matrix surrounding them, are locally known as 

 Potato-stones, is mentioned (Geol. Trans, vol. i. pp. 212, 291-294, 

 &c. ; ibid. 1st ser. vol. iv.). These cavities so filled, and shown to be 

 particularly abundant near Mells (Mendip Hill district), are of the 

 same kind as those noticed at Kenfig Point, Glamorganshire," where 

 the most westerly patch of conglomerate occurs. 



On p. 249, De la Beche notices intercalations of flesh-coloured 

 dolomitic limestones with the marls, on the south of the Mendips, 

 near Croscombe. They seem to be passage-beds into the underlying 

 dolomitic conglomerate. The presence of arenaceous matter in the 

 lower part of the Trias, in neighbourhoods where Palaeozoic sand- 

 stones occur, has been already noticed; it might be of interest, 

 however, to quote an instance at Portishead, given by De la Peche 

 in the work before referred to (pp. 248, 249 *), the beds being 

 given in descending order : — 



1. Conglomerate. 



2. Layers of red and buff-coloured sandstone. 



3. Conglomerate. 



4. White crystalline dolomite. 



5. Buff-coloured sandstone. 



6. Conglomerate reposing on the edges of the coal-measure sandstone 



(Pennant grit). 



Iii fact, throughout this district, the general absence of arenaceous 

 matter of Triassic age may be accounted for by the great quantity 

 of calcareous sediments afforded to the denuding agents of that 

 period, through the prevalence over the area of Carboniferous lime- 

 stones. Even in cases where the shores were wholly composed of 

 Carboniferous sandstones the transport of calcareous matter from 

 adjoining limestone coast-lines prevented the representation of the 

 marginal deposits by sandy beds alone, but produced an intercalation 

 of sandy and calcareous matter, according as the degradation of the 

 sandstone cliffs was counteracted by the interception of beach 

 materials, or counterbalanced by the influx of calcareous sediment 

 from the vicinity. 



The reverse is the case in the Devon area, where the shales and 

 grits of the Devonian and Culm-measure strata afforded a mass of 

 arenaceous sediment, in which the occasional limestones occurring 

 in them were in their degradation unable to produce any marked 



* Vide Moore, " Abnormal Secondary Deposits," Quart. Journ. Geol. Soc. xxiii. 

 p. 456 : a gallery driven in search of manganese, through conglomerates with 

 intercalated masses of bluishand yariegated marl, at Croscombe. 



