1852.] PRESTWICH ON THE THANET SANDS. 235 



2. On the Structure of the Strata between the London Clay 

 and the Chalk in the London and Hampshire Tertiary 

 Systems. By Joseph Prestwich, Jun., Esq., F.G.S. 



Part III.* — The Thanet Sands. 



[Plates XV. XVI.] 



(Part II. is deferred until a later period for reasons assigned in the subjoined note.) 



The first part of this paper, containing a description of that portion 

 of the Lower Tertiaries immediately under the London clay, and which 

 I termed the ^'Basement Bed of the London Clay,'^ was read before 

 the Society in January ISoOf. On that occasion my object was to 

 show, that over the whole of the south of England tertiary area, a 

 stratum forming a distinct and constant geological horizon clearly 

 separated the London clay from the group of strata beneath it. The 

 variable and interesting set of deposits between the Chalk and the 

 London clay, including the " Basement Bed" of the latter, form the 

 group hitherto called " the Plastic Clay Formation," which has been 

 described " as composed of an indefinite number of sand, clay, and 

 pebble beds, irregularly alternating,"" and as being " members of one 

 great series of nearly contemporaneous deposits," and essentially of 

 fresh and brackish water origin. A careful examination of these strata 

 has led me to believe, on the contrary, that a regular and definite order 

 of superposition does exist, and that, instead of one series of alterna- 

 ting and intercalated strata, the conditions of structure and changes 

 in the fauna show that there are five well-marked and distinct groups. 

 Three, however, of these groups are apparently synchronous. There- 

 fore the number of consecutive and separate divisions of the Lower 

 Tertiaries may be reduced to three J. 



The term *' Plastic Clay Formation" was originally given to this 

 series in conformity with the divisions introduced into the French 

 tertiaries by Cuvier and Brongniart, and was deemed applicable in 

 consequence of the prevalence of variegated plastic clays at many 



* The difficulty of obtaining, in the very variable strata of the " Low^er Tertia- 

 ries," a series of sections that would, to my mind, satisfactorily establish their 

 di%'ision into certain well-marked and persistent groups, has caused a longer delay 

 than I could have wished in bringing this inquiry to a conclusion ; and even now 

 I find it advisable to describe the lowest member of the series before the central 

 one, that should in natural order have followed next, the better to detach out of 

 the series this latter more complex group, the description of which I must post- 

 pone to a future period. [J. P., Jan.] 



t Quart. Journ, Geol. Soc. vol. vi. p. 252. 



X It is true that certain characters and fossils pervade the whole series, and it 

 may be thought unnecessary to establish divisions on features not more distinc- 

 tive than those which guide us in these papers ; but if not standing out in the 

 strong relief indicating more important geological changes, they are, though small 

 in degree, definite in form, and exhibit the more delicate shadings, stamped as it 

 were by the diurnal occun-ences of the time. They may be less impressive than 

 the greater changes, but, as they are equally a part of the progress of the period, 

 and depict the more gradual and lesser alterations in the configuration of the sur- 

 face and in the distribution of hfe, they are full of interest and indispensable in 

 filling up those details of the scene, which are so necessary to enable us to trace 

 accurately each successive step in the history of the time. 



