1850.] AUSTEN SANDS AND GRAVELS OF FARRINGDON. 455 



actually visited : the line of country was selected by Mr. Prestwich, 

 and it may be as well to state briefly in the outset, the points of geo- 

 logical interest which mainly influenced the choice. 



Those who are acquainted with the memoir of Dr. Fitton on that 

 portion of the cretaceous series which occurs in this country below 

 the chalk, are av/are that the author clearly establishes that the 

 several deposits of upper greensand, gault, and lower greensand are 

 clearly distinguished, and developed in great thickness along the 

 S.W. coast of the Isle of Wight. Subsequent observers, by greatly 

 extending the knowledge we had of the lowest cretaceous fauna of 

 this region, and determining the range of the various forms, have 

 rendered it necessary that the base of this series should be every- 

 where re-surveyed. From the Isle of Wight westwards, the next 

 section of the beds below the chalk is that presented at Swanage Bay, 

 being thirty miles due west of the former : the difi'erences which 

 these two sections present are briefly these. The Wealden sands of 

 Swanage are vastly thicker than in the Isle of Wight ; the clays which 

 surmount the sands in the Isle of Wight are on the other hand 

 greatly thicker than those of Punfield, which are reduced to a few 

 feet, but in which Cyclades and Faludince are so abundant that there 

 can arise no question as to the extent to be assigned to them. 



The very remarkable fact established by the Swanage Bay section 

 is, that in an interval of thirty miles, the lower greensand, as ex- 

 hibited at Atherfield, should almost or entirely have disappeared : 

 beds which are the mud deposits of a freshwater lake are brought 

 within a few feet of the sea mud, known as gault, and Avhich here, 

 though the same fossil forms mark it, has lost much of its dimen- 

 sions, as compared with its extent in the Isle of Wight. 



The strata of the cretaceous beds at Swanage are inclined at a very 

 high angle ; and the comment which the author of this communica- 

 tion would make upon Dr. Fitton' s representation of the cretaceous 

 series, as it occurs at this place, is, that the admeasurements of the 

 beds are all greatly in excess, and that the lower greensand is not 

 represented there at all. 



In his representation of the cretaceous series, as exhibited in the 

 cliffs of the west of the Isle of Purbeck, Dr. Fitton very correctly 

 omits the lower greensand : as we proceed inland, it seems in like 

 manner to be altogether wanting as far as the Vale of Wardour ; at 

 this place Dr. Fitton hardly ventures to introduce it, nor does his 

 section or list of fossils from Warminster indicate its presence there. 



The lower greensand, as a distinct division, is not expressly stated 

 to occur until we reach North Wilts ; it is introduced in sections 

 Nos. 15 and 16 of Dr. Fitton's memoir *, and a list of fossils is given 

 at p. 268 ; but, with the exception of a section through Swindon, 

 Dr. Fitton does not profess to describe the succession of beds in de- . 

 tail, until he reaches Oxford. 



In this way, and avoiding the field of others' inquiries, we seemed 

 to have open to us a well-defined line of unexamined country ; and our 



* Geol. Trans, vol. iv. part 2. 2nd Series. 



