GIBB — OYSTER CONGLOMERATE BED AT BROMLEY. 329 



many of the oysters were strangers to the spot on which they are now 

 found, from the very great preponderance of single valves over the 

 perfect shell, as well as finding many of the latter in over-turned 

 positions. The following are the fossils which I obtained from this 

 excavation ; there are other species of Ostrea and Cerithium besides 

 those named ; — 



Ostrea bellovicina. Serpula vermicularis. 



edulina. Vermilia triquetra. 



Cyrena cuneiformis. Perforations of Clionidae. 



Cerithium variabile. Minute scales of fish. 



Pectunculus Plumsteadiensis. Indistinct traces of plants. 



Modiola Mitchelli. 



At the railway cutting, a quarter of a mile to the south-west of 

 Bromley, a section of the same deposits is to be seen running through two 

 high hills, consisting of gravel, layers of sand, and shell breccia, but 

 the last consists of small shells, with comparatively few oysters, and 

 not so many rolled flints at this spot. Throughout the whole of 

 Bromley, on digging for wells, this conglomerate has invariably to be 

 reached before water can be obtained; one well shown to me gave a section 

 of two feet of loam, two feet of gravel, twelve feet light grey and brown 

 sands, and seven feet of oyster-conglomerate ; a plentiful supply of water 

 had been reached at this depth. This occurs when the dry and hard 

 rock, especially at the surface of the conglomerate, has been cut 

 through. 



The Lower London Tertiaries are divided into three sub-groups by 

 Mr. Prestwich, the upper one, which is called the Basement-bed of 

 the London clay," is a marine deposit, resembling the beds beneath it 

 in mineral character, but in organic life more nearly related to the 

 London Clay above it. The middle sub-group, or the Woolwich and 

 Reading Series," is a very variable group of freshwater and sestuarine 

 origin. The lower sub-group, or the Thanet sand, is a marine bed of 

 much less extent than the other two. These three groups are ably 

 described in the third lecture of Mr. Prestwich' s, " The Ground Beneath 

 TJs."*' 



* Visitors to Bromley can avail themselves of the services of Mr. Porter, the 

 enterprising landlord of the " Rising Sun," who will undertake to pilot them to 

 the railway-cutting, the rock-pit, and the underground chalk-quarry. 



2 c 



