TRIAS, 231 



quarried, and here remains of Plants and Fishes have been 

 obtained.^ 



Red sandstones and marls, possibly of New Red Sandstone age, 

 have been met with in some of the deep borings near London. 

 (See p. 140.) 



TRIASSIC ROCKS OF SOUTH "WALES AND THE SOUTH-WEST 

 OF ENGLAND. 



In Glamorganshire, Gloucestershire, and in the neighbourhood 

 of the Mendip Hills in Somersetshire, the Red Rocks consist 

 of Dolomitic Conglomerate, Dolomitic Limestone, Sandstone 

 (generally very calcareous), and Marls. 



The Dolomitic Conglomerate, locally called ' Millstone ' or 

 ' Millgrit rock,' is an old beach-deposit of Keuper age, derived 

 chiefly from the Carboniferous Limestone. It occasionally contains 

 pebbles of Millstone Grit and Coal-measure Sandstones ; but rarely 

 any pebbles of Old Red Sandstone, partly because they are not 

 so extensively exposed along the old margins, and partly because 

 most of the sandstones would be too friable long to resist the 

 friction to which they were subjected. In thickness the Con- 

 glomerate is rarely more than 30 feet. The included fragments 

 are sometimes well rounded, but often so slightly worn as to 

 constitute a breccia rather than a conglomerate ; they vary in 

 size, from small stones to boulders two or three feet in diameter. 

 These are often cemented together by the carbonates of lime 

 and magnesia, whence the name Dolomitic or Magnesian Con- 

 glomerate ; but frequently the cementing material is simply car- 

 bonate of lime, marl or ferruginous sand ; the matrix is usually 

 much coloured by peroxide of iron.* 



The Dolomitic Conglomerate usually occurs at the base of the 

 Red Marl, and yet at the same time it occurs at all horizons along 

 the margin of that deposit, where the beds dovetail one into the 

 other, proving, as De la Beche originally pointed out, that its 

 formation continued throughout the entire series. (See Fig. 24, 

 p. 160.) 



Sir A. C. Ramsay regarded the Dolomitic Conglomerate not 

 merely as of aqueous origin, but as in part formed of breccias 

 which had covered the old land surface and had been worked up 

 by the waters of the New Red period.^ Mr. R. H. Valpy informed 

 me that he had found striae, which he considered might be of 

 Glacial origin, on blocks of Carboniferous Limestone in the Dolo- 

 mitic Conglomerate. Prof. W. J. Sollas also has noted striated 

 pebbles in the Conglomerate at Portskewet in Monmouthshire ; 



1 Rev. W. S. Symonds, Q.J. xi. 450. 



- The term "Dolomitic Conglomerate" was applied by Buckland and Cony- 

 beare, T. G. S. (2), i. 292. The term " Magnesian " applied to the conglomerate 

 has sometimes caused it to be considered of Permian age. It has also been ranked 

 as a possible equivalent to the Muschelkalk, R. Etheri'dge, Q. J. xxvi. 1S9. 



^ Q. J. xxvi. 191. 



