STRATA IN THE NEIGHBOURHOOD OF LONDON. 4| 



the strata, therefore, of this formation, these remarks cotn- 

 mence. 



Beds of Sand and C ravel. 



The sands of this formation vary in coiour from white, B; els of sand 

 which is most rare, through different sliades of \e!!ow up to anci^xAvd. 

 orange-red: the colour proceeding partly frotn a ferrno^in- 

 ous stain on the suface of the particles of s;iiid, and partly 

 from the intermixture of yellow oxide of iron. Particles of 

 those sands, which are disposed in distinct seams or beds, 

 when examined by the microscope, ui-e found to be transpa- 

 rent, most of them angular, but some a little rounded, with 

 all their surfaces smooth, havjug ;io appearance of fracture, 

 and resembling, in every respect, a uniform crystalline de- 

 position. Those sands on the contrary, which, blended with 

 broken and unbroken pebbles, foiin gravel, appear, when 

 thus examined, to be mostly otJacpie, to be variously colour- 

 ed, and to be marked with conthoidal dcpresMons and emi- 

 nences, the result of fracture. 



The peljbles of this formation appear to be of four kinds, rebW<^ of ;1h« 



1st. Various pieces of jasper, gritstone, white semitrans- j"^"'"'^'^'** 

 parent quartz, and other rocks. These have acquired, in 

 general, smooih surfaces and roundish ft.rms, evidently from 

 attrition, and exhibit no traces of organization, except when, its oj^Tiic re- 

 as is very rarely the c;ise, tlu' substance of the pebble is jas- "v.iw rarr,iini 

 perised wood. The white quartz pebbles, like quartz crys- °''^ ^''^'^' 

 tals, on being rubbed together, emit a strong white lambent 

 lightj with a red hery streak on the line of collision, and aa 

 odour whicli mtich resembles that of the electric aura. 



'2d. Oval, or roundish, and rather flat fciliceous pebbles, 2d kind, 

 generally surrounded by a crust or coat differing in colour 

 and tlegrce of transparency from the internal substance, 

 which also varies in different specimen^, in trese respects, as 

 well as in the disposition of the parts of which the substance 

 is composed. In some this is spotted, or clouded, in ver^ 

 beautiful forms; in others it is marked by concentric striae, 

 as if the result of the successive application of distinct lami- 

 nae: the prevailing colours in most of these pebbles beij»g 

 different shades of yellow. In several the traces of marine Marine tp- 

 remains are observable: these are, in some, the casts of "'"^''V'^""*^ ^'^ 



' several. 



anomice, 



