L 



[ 264] 



L N 



masses, called septaria, are some- 

 times continued through a thick- 

 ness of two hundred feet ; of these, 

 Parker's cement is made. The 

 septaria lie horizontally, and are 

 disposed at unequal distances from 

 each other in seemingly regular 

 layers. They frequently include 

 portions of wood pierced by tere- 

 dines, nautili, and other shells ; 

 and it is a curious fact that septa 

 of calcareous spar frequently inter- 

 sect the substances contained in 

 the septaria. These septaria were 

 at one time deemed characteristic 

 of the London Clay, but they have 

 been found in other formations, 

 more particularly in the upper 

 parts of the "Wealden. Sulphuret 

 of iron, phosphate of iron, and 

 selenite are found interspersed 

 throughout the London Clay ; on 

 which account the water issuing 

 from it is not fit for domestic pur- 

 poses. Amber and fossil copal or 

 resin have been found in this 

 deposit. From the London Clay 

 three or four hundred species of 

 testacea have been procured, but 

 the only bones of vertebrated 

 animals are those of reptiles and 

 fish. Remains of turtles have 

 been dug out of this deposit at 

 Highgate and Islington, and some 

 bones of a crocodile were discovered 

 by Mr. Parkinson ; nautilites also 

 are found in it. The shells of the 

 London clay mostly belong to gen- 

 era inhabiting our present seas. 

 The following is principally ex- 

 tracted from Conybeare and Phil- 

 lips's Geology of England and 

 Wales. 



An idea of the nature and com- 

 position of the London Clay forma- 

 tion may be arrived at by the fol- 

 lowing section afforded by a well 

 sunk at Tottenham, in Middlessex. 

 1. Immediately below the sur- 

 face was found brick earth and 

 coarse yellow sand, stiff lay, 

 and marl. 20 



2. Blue clay of various inten- 

 sity of colour and degrees of 

 stiffness, adapted for tile-ma- 

 king. It effervesced slightly, 

 and enclosed hard and irregular 

 masses of a lighter colour, full 

 of minute appearances of char- 

 red vegetable matter, and sep- 

 taria, which also effervesced. 60 



3. Blue clay of a greasy aspect 

 and somewhat greasy to the 

 touch ; it did not effervesce. 20 



4. Purple, blue, red and brown 

 clay mixed, having greatly the 

 appearance of some varieties of 

 lithomarge; it did not effer- 

 vesce. 10 



5. Blue, white, and brown clay 

 mixed, much heavier than the 

 preceding : it contained very 

 compact, hard, and nearly 

 cylindrical masses, six to 

 twelve inches long, and of a 

 yellowish- white colour : it effer- 

 vesced strongly. 10 



6. Yellowish white clay, fre- 

 quently compact and hard, and 

 equally heavy with the prece- 

 ding : it effervesced strongly. 3 



7. Rock bored through. 2 



Feet, 125 



There are very few genera of 

 recent shells which have not some 

 representative imbedded in this 

 formation, but the specific charac- 

 ter is usually different; on the other 

 hand but few of the extinct genera, 

 so common in the older formations, 

 occur in this, so that it seems to 

 hold a middle character in this 

 respect between the earlier and 

 more recent beds. Thus though 

 nautilites resembling those of the 

 Indian seas are common, specimens 

 of the cornu ammonis and the 

 belemnite are so rare, that it is 

 very doubtful whether they have 

 ever been found. Echinites, so 

 common in the chalk, are very rare 

 in this formation. 



The most interesting facts con- 



