OF THE MEUX-WELL DEPOSITS. 921 



organic remains. In giving consideration to those which have been 

 derived from the lowest 40 or 50 ft. above the Devonian, there ap- 

 pears little doubt that two fades can be recognized palseontologically, 

 and I think, also (though there may not be much difference), as to 

 condition, one group, and that the largest, will prove to be marine, 

 and probably of the age of the Lower Greensand ; the other group 

 from fresh or brackish water, and though now obtained from the 

 same deposit, implying that when living they must have been subject 

 to different physical conditions, what those were being now only 

 matter for speculation. The marine forms appear, though dismem- 

 bered and fragmentary, to have been the least subject to erosion. 

 Had they all been littoral or estuarine they would have been liable 

 to the same amount of attrition ; and it seems impossible that the 

 delicate shells of the marine Microzoa could have undergone the 

 rough treatment of some of the other remains without having been 

 destroyed ; the Entomostraca especially come out as detached shells, 

 or in single valves, as sharp as when living. 



I am most inclined to the view that the surface of the Devonian 

 beds under London was occupied by shallow lacustrine basins, their 

 waters having sufficient mechanical movement from streams or other- 

 wise to cause the erosion of its many organisms. In the open 

 quany sections at Swindon of the freshwater Purbecks it may be 

 seen, from the channelled character and want of continuity of some 

 of the beds, that they must have been subject to repeated movements. 

 Probably this was so with the beds under notice, until they were 

 invaded and at last finally occupied by the seas of the Neocomian 

 period. In this case the lacustrine deposits must have preceded 

 those of the Lower Greensand, though probably not by a long inter- 

 val. The precise stratigraphical horizon the former would occupy 

 is, under the circumstances, difficult to determine. 



Some difficulties have been experienced, and considerable time 

 occupied, in working out the details of this necessarily imperfect 

 paper. No doubt many interesting geological studies would present 

 themselves along the line of the Palaeozoic barrier as it passes 

 onwards from the Mendips to the east at points where it is met by 

 later deposits, could its surface be uncovered. In a former paper 

 I pointed out that the elevatory force which had so great an effect 

 upon the uplift of the Mendips was exercised from the south of the 

 line of elevation during Triassic times, whereby the Palaeozoic beds, 

 including the Coal-measures, were carried forward to the north, 

 their elevation in this direction preventing so thick a deposition 

 of Secondary beds thereon; whilst it was shown by sections that 

 there was a corresponding trough-like depression on the south, 

 whereby the Secondary rocks were thickened, a fact which was pro- 

 bably fully illustrated by the experiment at Battle. 



It is to be hoped that, on the occasion of another deep boring, 

 the northern slopes of the Palaeozoic barrier may be fortunately 

 struck, and persevering scientific research have its full reward. 



