244 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 21, 



the chalk, of a layer of flints of all sizes, just as they occur in the un- 

 derlying chalk, from which in fact they appear to have removed com- 

 paratively without wear or fracture ; for they are almost as perfect as 

 the undisturbed flints, but present this difference, that instead of their 

 usual white or black coating, these flints are almost invariably of a deep 

 bright olive-green colour externally ; the white outer coating, which 

 is often very thick, seems removed (as though by an acid), and the 

 flint then stained green. So strong is the colour, although it forms 

 a mere film, that flints removed by denudation from this bed, sub- 

 jected to great wear and many changes, and imbedded in fresh beds, 

 whether of the Tertiaries or the Drift, can always be recognized by 

 the peculiar green colour which they invariably retain. The colour 

 in fact seems to be not a mere stain, but an actual alteration in the 

 structure of the flint, arising apparently from its having entered into 

 a chemical combination with the iron of the mud or silt in which 

 they became imbedded, forming in consequence a true silicate of 

 iron. There is frequently a thick inner brown stain of the peroxide 

 of iron, but as in the silicate the iron is usually in the state of the 

 protoxide, it would almost appear as though the flints had been im- 

 bedded at the sea-bottom in a ferruginous mud, and that then some 

 cause, productive of an action on the silica, and a decomposition or 

 deoxidization of the mineral containing the iron, acted simultaneously 

 on the two, and brought them, in presence, into a state in which they 

 would readily combine. To this however I merely direct attention ; 

 it is a subject which needs a special inquiry. If it should prove to 

 be, as I anticipate from the few experiments I have made, a true 

 chemical combination, it will be a curious fact, for almost all the 

 minerals in which the silicate of iron is a main ingredient, as the 

 chlorites, amphiboles, pyroxenes, &c., belong to rocks of igneous or 

 metamorphic origin. Its formation by moist means seems to be the 

 exceptional case ; but should the view here suggested be correct, 

 it will show that there may be cases in which sedimentary beds 

 of greensand may have derived their characters from changes sub- 

 sequent to their deposition, as well as by the more usual direct dis- 

 integration of the unstratified and metamorphic rocks. 



There is on the whole in the Thanet Sands a uniformity and 

 breadth of character entirely wanting in the overlying division of the 

 Lower Tertiaries, which deposit is on the contrary extremely variable 

 in its structure, showing rapid changes within short distances and 

 great variety of lithological composition. This well- maintained re- 

 gularity in the one, whilst the accompanying overlying beds undergo 

 at the same time considerable alteration, is in fact one of the features 

 which serves to mark the two divisions, notwithstanding that the 

 middle group, in its many phases, occasionally puts on characters so 

 resembling the lower ones as to render it diflicult to distinguish them 

 apart. This fact will, however, be brought out more clearly in descri- 

 bing hereafter the next or middle division of the Lower Tertiaries. 



For a more particular account of the lithological characters of this 

 deposit at a few given places, see the descriptions of sections, p. 250, 

 and Explanation of Plate, p. 261. 



