and Fossil Reinains near London. 329 



cliaracters, being found together in particular spots. Thus those 

 in the County of Essex, ten miles northward of London, contain a 

 much greater proportion 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 Egj^ptian 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 veiy 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, 

 €0mewhat resembling in figure the flints which are found in ch^^ilk, 

 materially differing however from them, not only in the colour of 

 flieir external coat which is of various shades of brown ; but also in 

 that of their substance, which is seldom black, but exhibits shades of 

 yellow or brown, in which red likewise is sometimes perceptible. 

 The traces of organic structure, particularly of the alcyonhcm occa- 

 sionally seen in these stones, determine them also to have been, 

 formed at the bottom of the sea. 



4th. Pebbles, owing their form to an investment and impregnation 

 *with silex, of various marine animais of unknown genera, but bearing 

 a close affinity to the alcyon'ia. These stones display, in general, 

 not only the external form, but the internal structure also of these 

 animals. The congregation of many pebbles of this genus, and 



* The gravel pebbles of Eppiiig Forest arc of this description ; and on most of th« 

 grounds leading down from the forest to the hamlet of Sewardstone and to the h)wn of 

 Waltham, -white, opaque, and partly decomposed pebbles are frequently seen, iu 

 which the argil and iron have bceu removed, and the silcx only has remaiucd. 



2t 



