470 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JuilG 19, 



lapse of time, hundreds of feet of sedimentary deposits, and altered 

 conditions of the whole of the northern hemisphere, are recorded 

 between those two periods. 



When we employ the term "lower greensand," we mean only to 

 designate deposits of sands and sandstones, occurring beneath a por- 

 tion of the cretaceous series, and connected with that series, not by 

 any specific identity of its organic forms, but by a certain general 

 resemblance and aspect of its fauna, and by some of its forms being 

 referable to the same divisions and subgeneric groups, as in its Fishes, 

 Cephalopods, and Echinoderms. 



The lower greensand is not everywhere co-extensive with the cre- 

 taceous series, and where it is wanting the series commences with 

 the gault, the chloritic sands, or even with the lower chalk, as the 

 case may be ; but whenever the lower greensand does occur, its re- 

 lation to the gault is invariable — it is beneath the gault*. It is not 

 necessary that we should here enter on an inquiry as to what were 

 the particular submarine conditions which caused the gault clays to 

 overlap the lower greensand in the extension of the cretaceous series 

 westwards ; a process which, in turn, is repeated by the upper green- 

 sand with reference to the gault ; but it is clear that so far as each 

 of these divisions extended itself, it must have done so continuously. 

 Such is not at all the condition of the masses of ironsand and gravel 

 in question; the Farringdon beds, with a thickness perhaps of 

 1 00 feet, rest, as an isolated mass, upon the Kimmeridge clays : at a 

 distance of two miles, the cretaceous series commences with the gault, 

 also resting on Kimmeridge clay. The sands and gravels at Far- 

 ringdon are thicker than at any other place at which we find them, 

 and did they belong to the cretaceous series, as represented by Dr. 

 Fitton, we surely ought to find some traces of them below the gault. 

 The remarks which arose on the subject of the isolated condition of 

 the masses of Portland, at Swindon and Bourton, apply with even 

 greater force to the much thicker accumulations of Farringdon : as 

 in that case, so here, we may feel assured that the beds at Farring- 

 don, Calne, and Devizes, formed part of a continuous zone of deposits, 

 which must have been denuded and reduced to isolated patches before 

 the period of the gault. Unless we can believe that these separate 

 masses severally had not at any time a greater horizontal extent than 

 they have at present, they must have been reduced to their present 

 dimensions before the gault could possibly have been deposited on 

 beds of Kimmeridge clay ; and if at Farringdon a long period of de- 

 nudation and removal of materials separates the ironsands and gravels 

 from the gault, we cannot suppose that they can possibly form toge- 

 ther a continuous and ascending series of deposits at Devizes. 



The position of the whole of the Farringdon mass on the Kim- 

 meridge clay, and its perfect independence of the cretaceous series, are 

 facts which rest on the evidence of sections respecting which there 

 is no ambiguity whatever ; and we may therefore feel assured that 

 they were subjected to that process of denudation which abraded the 



* See Dr. Fitton's Sections, joamm. 



