6 LONDON DISTRICT. 



It will be seen from the table that the depth below Ordnance 

 datum to the Palaeozoic rocks varies but little from 1,000 feet. 

 The form of the Palaeozoic platform has recently been discussed 

 by Sir A. Strahan, 1 who showed that the contour of 1,000 

 indicates a slight ridge running north-west to south-east, under 

 London, from which the levels fall in all directions except to 

 the north-west. If, however, allowances are made for subse- 

 quent earth-movements, the shape of the platform when newer 

 rocks were laid down against or over it is found to be different ; 

 the northern half of our area is high ground falling away rapidly 

 to the south and gradually to the north-west (Plate C). There 

 is as yet insufficient evidence to determine the strike of the 

 ancient rocks; at Ware, 9 J miles north of our boundary, and 

 again at Cliffe, 12 miles east of Dartford, Silurian rocks underlie 

 the Cretaceous, while at Bobbing, 23 miles east-south-east of 

 the same town 156 ft. of Jurassic strata intervenes. So far as 

 is known, the intermediate area is occupied by the Devonian 

 rocks. 



JURASSIC ROCKS. 



GREAT OOLITE. 



Bocks belonging to the Great Oolite series have been defi- 

 nitely recognised beneath London. At Richmond, 87 J ft. of 

 shelly and oolitic limes tones with a band of fissile sandstone, 

 has been grouped with the Great Oolite. The series was entered 

 at a depth of 1,151 ft. from the surface. At Streatham, at 

 a depth of 1,081 ft., 38 J ft. of oolitic limestones, clays and 

 sandy beds may be assigned to the Fuller's Earth Rock and the 

 upper part of the Great Oolite. At Meux's Brewery, at a depth 

 of 1,002 ft., 64 ft. of limestones, some beds oolitic, with a 

 seam of sand and a few quartz pebbles has been grouped with 

 this series. Coaly fragments have been found in the strata, 

 but these, in Professor Judd's opinion, might have belonged to 

 1 actual Jurassic coal seams.' 2 



Thus we find, at depths of 1,000 feet and more under London, 

 strata, the representatives of which come to the surface about 

 100 miles distant on the west. There is evidence, however, 

 that the Jurassic strata occur under London in the form of a 

 denuded anticline, as on the west and north-west the Kimmeridge 

 Clay, Portland and Purbeck beds are the nearest of the exposed 

 Jurassic rocks. On the east beneath Chatham, the Oxford 

 Clay has been reached, and on the south-east higher Jurassic 

 divisions occur. Over most of the central area, as may be seen 

 in the table, these rocks are absent. 



CRETACEOUS ROCKS. 

 LOWER GREENSAND. 



The Lower Greensand is of great importance as a source of 

 water supply, but it is absent over the greater part of the area. 



1 Quart. Journ. Geol. Soc., vol. Ixix, 1913, pp. Ixx-xci, contour map, Plate A. 



2 Quart. Journ. Geol. Soc., vol. xl, 1884, pp. 748, 749. 



