and Fossil Remains near London. 327 



stances great difficulties and confusion frequently arise- in examin- 

 ing the superior strata j the counties however immediately surround- 

 ing the metropolis, as well as that on which it stands, having suffered 

 least disturbance, are those in which an investigation of these strata 

 may be carried on with the smallest chance of mistake. 



Real alluvial fossils, washed out of lifted or original superior strata 

 'by strong currents, and which in other parts are very abundant, 

 are rarely seen in the counties adjacent to the metropolis. This re- 

 mark is rendered necessary, since those widely extended beds of 

 sand and gravel, with sandy clay, sometimes intermixed and some- 

 times interposed, and which have been generally hitherto considered 

 as alluvial beds, are here assumed to be the last or newest strata of 

 this island, slowly deposited by a pre-existent ocean ; with the strata, 

 therefore, of this formation, these remarks commence. 



Beds of Samd and Gravel. The sands of this formation 

 vary in colour from white, which is most rare, through different 

 shades of yellow up to orange-red ; the colour proceeding partly from 

 a ferruginous stain on the surface of the particles of sand, 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, are found to be transparent, most of them angular, but 

 some a little rounded, with all their surfaces smooth, having no ap- 

 pearance of fracture, and resembling, in every respect, an uniform 

 crystalline deposition. Those sands on the contrary, which blended 

 with broken and unbroken pebbles form gravel, appear, when tlius 

 examined, to be mostly opaque, to be variously coloured, and to be 

 marked with conchoidal depressions and eminences, the result of 



fracture. 



The pebbles of this formation appear to be of four kinds, 



1st. Various pieces of jasper, gritstone, white seriii-transparent 



quartz, and other rocks. These have acquired, In general, smooth 



