1867.] BRISTOW LOWER LIAS OP GLAMORGANSHIRE. 201 



not only mixed with them mechanically in the form of fragments 

 of chert derived from the neighbouring Carboniferous Limestone, 

 but as a contemporaneous deposit. One of the lower hard beds in 

 the upper part of the series has an interrupted and irregular, but 

 very persistent and marked, layer of black chert near its base. 



The whole of the beds np to the acknowledged Lias may be de- 

 scribed as more or less conglomeratic, the uppermost 11 feet under 

 Southerndown being notably so. And, as Mr. Tawney states (at 

 p. 74), this latter stone differs lithologically from that of Sutton 

 in the beds being much harder and more irregularly [? regularly] 

 bedded. 



The whole thickness of these conglomeratic beds, from the normal 

 Lias down to the Carboniferous Limestone, is displayed in the cliffs 

 on either side of the caves already mentioned. The annexed dia- 

 gram * has been constructed from very careful measurements of the 

 cliffs, bed by bed, and checked by measurements made at different 

 times and at different points. The thickness of the beds varies 

 shghtly in different places, as is usually the case with deposits formed 

 in an area that is undergoing slow depression ; and the lower part 

 is especially irregular, conforming itself to, and filling the hoUows 

 and irregularities in, the denuded surface of Carboniferous Limestone, 

 upon the upturned edges of which it has been deposited. After the 

 cliff-section had been measured bed by bed, with the assistance of 

 Mr. Gibbs, of the Geological Survey, I measured the vertical thick- 

 ness of the beds, between the Carboniferous Limestone and the 

 lowest thick bed, in one measure (at a spot about | an inch east- 

 ward of the left-hand extremity of Mr. Tawney's longitudinal section, 

 fig. 2), and found it to be about 25 feet, certainly not more, if so 

 much ; and this measurement was tested at the caves by means of 

 another, with which the former closely agreed. 



This, according to my measurements, makes the total thickness 

 of the conglomeratic beds (the Sutton and Southerndown series of 

 Mr. Tawney) to be from 35 to 37 feet ; while Mr. Tawney represents 

 the aggregate thickness of the same beds to be 89 feet 9 inches. 



From the high ground of Sutton the beds gradually decline east- 

 ward, until at the caves they come to the base of the cliffs, a short 

 distance beyond which they are partly concealed from observation 

 in consequence of an undulation in the Carboniferous Limestone, 

 until they reappear at Dunraven Point, where the latter is brought 

 up by a fault. 



As the Sutton Stone reposes on the upturned edges of the Carbo- 

 niferous Limestone, the irregularities and undulations of which it 

 fills up, the thickness is somewhat variable in different places ; and 

 it is possible that the lower beds of Mr. Tawney's vertical section, 

 above and below the " Conglomeratic band of chert," which I believe 



* An attempt is made to show, in profile, the way in which the beds have 

 been acted upon by the sea. The upper part of the section represents the lower 

 part of the cliff under Southerndown — the remaining the subordinate portion 

 of the series aa displayed further west, down to the Carboniferous Limestone. 



