468 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Juiie 19, 



Bourton (10 ft.), a thickness which impHes a vast lapse of time: 

 conglomerate beds occurring in this way only indicate that the depth 

 of this portion of the oolitic series, at this particular spot, was that 

 of the zone along which subangular pebbles are drifted. The cal- 

 careous portions of the two masses do not differ to any extent worth 

 noticing ; and the yellow and buff fine-grained sands, which at 

 Bourton surmount the calcareous oolitic sands, taken together, corre- 

 spond in thickness with the main mass of sands and calcareous sand- 

 stones at Swindon, whilst the uppermost calcareous beds of Bourton 

 would agree in position with the main portion of the limestone, the 

 rest of the series haiing been removed from above the Bourton mass. 

 It will be seen from this that the Portland beds at Swindon and 

 Bourton are isolated masses of considerable thickness and compact 

 structure, resting on Kimmeridge clay. The surface of the oolite 

 formation may have been eroded at various times, but the effect pro- 

 duced at one particular period is perfectly distinct from every other. 

 No fact can be more certain than that the sea cannot transport sand 

 and gravel across areas of impalpable deep sea mud, and there pile 

 them up in detached conical masses. So that when we meet with 

 such accumulations as those of Bourton and Swindon, we may 

 feel assured that at some time they formed portions of continuous 

 beds of such materials, and having an extent equal, at least, to the 

 area over which the isolated patches now occur. The fact that these 

 Swindon and Bourton masses of the Portland series rest on Kimme- 

 ridge clay, and that this same clay, at short distances, varying from 

 half a mile to two miles in breadth, passes beneath the gault, clearly 

 shows that this particular denudation of the Portland had taken 

 place before the earliest beds of the cretaceous series were deposited. 



§ 3. Devizes. 



It will not be necessary to notice the beds of upper greensand 

 which occur here, as they are well known from the forms which they 

 supplied to the ' Mineral Conchology ' : they attain a great thickness, 

 are well exhibited in several deep road sections, and are more uni- 

 formly arenaceous than at places to the eastward — a mineralogical 

 change which becomes marked as the upper greensand ranges from 

 east to west. 



About halfway down the series of rocks at this place, beds of blue 

 clay are exposed in a brick-yard, and which were long since described 

 as "gault" by Mr. Lonsdale. The clay descends to the level of the 

 water at the bridge, which would give it a thickness of about 40 feet : 

 just beyond the bridge, beds of thin-bedded sandstone are exposed 

 in the bank of the Canal, and like beds, together with ferruginous 

 bands, were traversed in sinking a well at the residence of the en- 

 gineer, close to the bridge : the ferruginous blocks were very fossi- 

 liferous, containing casts of a small Nucula in great abundance. Area, 

 Cijpricardia, and Emarginula. 



From the information we received from Mr. Cunnington, it would 

 appear that the thickness of this bed of ironstone, sandstone, and 



