448 Mr. Austen on the 



fig. 8 A.) The proportion which the crj^staUine rocks of the upper Blackdown beds 

 bear to the rest, is less than in the equivalent ones of Haldon, but the pebbles are 

 mineralogically identical, and undoubtedly have been derived from the same quarter. 



In these beds we seem to have the last operation in the order of events, before 

 the excavation of the deep valleys of the district commenced. 



A bed of considerable thickness, composed chiefly of angular chalk flints, but 

 containing in its lower portions large tabular and angular blocks of chert and sand- 

 stone, mixed with sand, is found on Blackdown and its ramifications, resting occa- 

 sionally on chalk, but more generally, as on Haldon and Milber Down (PI. XLI.), 

 on greensand. This accumulation lies on a very uneven surface, as is well exhibited 

 along the coast-section generally, both east and west of Sidmouth ; the depressions 

 which produce this rugged outline being either troughs, or deep, inverted cones. In 

 1836 I examined one of the latter, which had been just emptied of the flints it had 

 contained. The strata in which it occurred, were of very compact sandstone, and 

 the sides of the pit, which was circular, exhibited deep concentric grooves, such 

 as would be produced by the circular motion of the materials within. Similar pits 

 have been often noticed over the surface of the chalk, and they are very common 

 in this neighbourhood. They have been attributed to the erosive action of acidu- 

 lous waters ; but as they are not confined to calcareous beds, extending here into 

 arenaceous strata, such an explanation can hardly be received as a general mode 

 of operation, and we must have recourse to the mechanical action of hard substances 

 set in motion by water, in the same manner as pits and basins are now constantly 

 produced along the coast and in rapid rivers. This mass of purely cretaceous 

 materials, the flints, coated by a black substance, being intermixed with abun- 

 dance of strong clay, is very distinct as an accumulation from the overlying water- 

 worn flints and pebbles of older rocks ; but it corresponds exactly with those beds 

 which in many places constitute the lowest tertiary deposits, where they rest on 

 chalk. In the Bovey valley a similar accumulation is subjacent to the pipeclay beds. 



Other portions of the tertiary series apparently once extended here. Scattered 

 largely over the surface of all this district, and mixed with the debris on the hills, 

 are blocks of a breccia, composed of angular fragments of chalk flints, cemented 

 together by an exceedingly hard, siliceous paste. This breccia affords proof of a 

 long post-cretaceous period of tranquil deposition, and of a subsequent one of de- 

 struction, of both of which it is the sole remaining indication. Besides the breccia, 

 there are large slabs, composed partly of similar materials, and in part (taking the 

 blocks according to their thickness) of a compact, fine-grained sandstone, some 

 blocks containing only an occasional flint, but some none at all, in which cases 

 they are mineralogical grey- wether sandstones ; and may probably be the equi- 

 valents of those siliceous masses, warranting, a presumption at least, that tertiary 



