1 34 On some of the Strata in the Neighbourhood of Londorif 



the casts of onmnirs, and the impressions of the spines and 

 plates of ech'nii; and in others, which generally possess a 

 degree of transparencv, the remains of ulcyonia. The im- 

 pressions, though frequently on the surface of the pebble, 

 seldom, if ever, appear to be in the least rubbed down : 

 thus seeming to prove decidedly, that these pebbles have 

 not been rounded by rolling, but that they owe their figures 

 to the circumstances under which they were originally 

 formed: it is apprehended therefore, that these pebbles have 

 each been produced by a distinct chemical formation, which, 

 it may be safely concluded from the remains of marine ani- 

 mals so frequently found in them, took place at the bottom 

 of the sea, while these animals were yet living. 



The formation of these fossils at the bottom of a former 

 sea, and perhaps on the identical spots in which they are 

 now frequently found, is more plainly eviiiced by pebbles 

 agreeing in some peculiar characters being found together 

 in particular spots. Thus those in the county of Essex, ten 

 miles northward of London, contain a inuch greater pro- 

 portion of argil and iron than those met with in many other 

 places; hence their colours are darker, and the delineations 

 which their sections display are very strong and decided, 

 sometimes closely agreeing with those seen in the Egypliati 

 pebbles*. Passing on into Hertfordshire, pebbles of a very 

 different character are found: their crust is nearly black, 

 and their section displays delicate tints of blue, red, and 

 yellow, disposed on a dead-white ground in very beautiful 

 forms. In another part of the same county occurs the 

 pebble of the pudding-stone, which also presents peculiar 

 characters of colour, &c. 



3d. Large tuberous, or rather ramose, irregularly formed 

 flints, somewhat resembling in figure the flmts which are 

 found in chalk, materially dift'ermg however from them, 

 >iot only in the colour of their external coat, which is of 

 various shades of brown, but also in that of their substance, 

 which is seldoin black, but exhibits shades of yellow or 

 brown, in which red likewise is sometimes perceptible. The 

 traces of organic structure, particularly of the alcyoniiirn, 

 occasionally seen in these stones, determine them also to 

 have been formed at the bottou) of the sea. 



4th. Pebiiles, owing their form to an investment and 



* The gravel pebbles of Epping Forest are of thin description; and on 

 mcit of the grounds leadine down from tlie forest to tl'.e hamlet of Seward- 

 sione and to the town of Waltham, wliite, opake, ai)d partly decomposed 

 pebbles are frecju.'ntly seen, in which the argil aud iron have beeu removed, 

 knd the silex ouly has remained. 



impregnailoa 



