

bearing on theoretical Speculations. 1 93 



mation approaches calcareous chains (as along the Mendips, 

 on the Bristol Avon, on the south edge of the South Welsh 

 coal basin, &c.)> its materials have been derived from the car- 

 boniferous limestone. 



In this case the blocks included are often of considerable 

 size, as especially in the section exhibited by the new road 

 ascending Clifton Downs from the end of St. Vincent's Rocks: 

 here they must sometimes weigh several tons. The pebbles 

 exhibit all the characteristic organic remains of the original 

 rock, and the outline of those remains is often truncated by 

 their rounded surface. 



This conglomerate has most evidently originally formed 

 beds of gravel lying against the chains whence that gravel was 

 derived, in a manner exactly similar to that of an actual sea- 

 beach ; they are thickest at the nearest points to those chains, 

 and regularly decrease as they recede from them. Thus the 

 calcareous conglomerate covering the coal-measures near the 

 Mendip chain is twenty-three fathoms in thickness ; about ten 

 miles distant from that chain it does not exceed one or two 

 fathoms, and still further off quite disappears. These deposits 

 forming the lowest members of the nearly horizontal strata 

 (the comparatively undisturbed position of which shows that 

 no material changes of relative level have been occasioned by 

 subsequent convulsion), we may confidently infer from the 

 superior level of the superincumbent lias, oolites, &c. that these 

 gravel beds were distributed beneath the then sea-level; and 

 the extreme dislocation of the subjacent carboniferous rocks, 

 &c. will sufficiently account for violent diluvial currents in the 

 then ocean. When we examine points where sections are pre- 

 sented of these conglomerates resting on the older rocks, 

 we find the edges of the strata of the latter truncated and 

 smoothed, and their surface often irregularly excavated, in a 

 manner which we cannot but ascribe to the operation of such 

 currents. 



The Rothe todte liegende of the German geologists is ge- 

 nerally identical in age and position with the conglomerates 

 now considered. I can at least answer that this is the case 

 with those portions of it which I have myself examined in the 

 neighbourhood of the Thuringerwalde. 



3. We do not meet with any other extensive accumulation 

 of water- worn pebbles in the strata, until we arrive at the ter-> 

 tiary deposits next above the chalk (the gravel associated with 

 plastic clay) : we indeed, in the intermediate formations, find 

 some traces of this kind ; for instance, in the calcareous grit 

 beneath the coral rag, and in the iron-sand below the chalk ; 

 but they cannot be compared with the conglomerates before 



N.S. Vol. 9. No. 51. Mar. 1831. 2 C described, 



