42 STRATA IN THii NriGHBOURllOOD OF LONDON. 



anomiie, and the impressions of the spines and plates of 



echini; and in others, which generally possess a degree of 



The])ebbles transparency, the renuiins'of alcyonia. The impressions, 



not rounded though freqvientlv on the surface of the pebble, seldom, if 



by rolling. , " • i i ■ , i , i 



ever, appear to be in tne least rubbed clown; thns seeming 



to prove decidedly, that these pebWes have not been round- 

 ed by rolling; but that they owe their tiguresto the circum- 

 stances under which they were originally formed : it is ap- 

 prehended, therefore, that these pebbles have each been 

 produced by a distinct chemical formation, which, it may 

 be safely concluded from the retnains of marine animals so 

 frequently found in them, took place at the bottom of the 

 sea, while these animals were yet living. 

 Pebbles of si- The formation of these fossils at the bottom of a former 



miijr charac- ^^^^ nerhaps on the identical spots, in which they are 



ters tounu to* * "^ , . "^ . •' 



igether. now frequently found, is more plainly evinced by pebbles 



agreeing in some peculiar characters being found together in 

 particular s[)ots. 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, some- 

 limes closely agreeing with those seen in the Egyptian 

 peb!)le*. Passing on into Hertfordshire, pebbles of a very 

 different character are found ; their crust is nearly black, 

 5ind their section displays delicate tints of blue, red, and 

 Tellov/, disposed on a dead-white ground in very beautiful 

 forms. Jn another part of the same county, occurs the peb- 

 ble of the pudding-stone, which also presents peculiar 

 characters of colour, &c. 

 jiiknc! 3d. Large tuberous, or rat"her ramose, irregularly formed 



Finn*, differ- fljjjts, somewhat resembling in figure the flints which are 



ftiM fiom those .... 



mchj,lk. found in chalk, materially differing however from them, 



not only in the colour of their external coat, which is of 



various sliapes of brown ; but also in that of their sub- 



* The pavcl pebbles of Epping Forest arc of this description ; and 

 on intist of the grounds leading down from the forest to the hamlet of 

 Scwardstone, :uul to the town of Walthara, white, opaque, and partly 



.decomposed pebbles are frequently seen, in vhich the ^rgil.and iron 

 \\9.\ti been removed, ai:d the silex only has r«uaiued. 



' ' stancCji 



