532 



can refuse to admit, that there have been more than one diluvial 

 action, since the deposition of the tertiary formations. In Norfolk, 

 he adds, it is true, the crag is involved in the clay, but if this clay 

 which in that county is 400 feet and in Suffolk 300 feet thick, be one 

 with the crag, it is most curious that a line of demarcation should 

 actually exist, between the districts in Suffolk occupied by tliese de- 

 posits ; and that the clay is never found below or intermixed with the 

 crag. Moreover this diluvial clay has been traced not only into Nor- 

 folk but into Cambridgeshire and Essex, close up to the metropolis. 

 In Suffolk the same line which bounds the London clay bounds the 

 diluvial. By an extension of Mr. Lyell's argument all diluvial depo- 

 sits, the author observes, might be included in the crag, and all other 

 formations considered as diluvial. The only rational conclusion in 

 Mr. Clarke's opinion, is, that during the crag era an extraordinary 

 convulsion took place which shook the whole country. He gives also 

 one or two instances in whicli diluvial clay and gravel, have been in- 

 troduced into cavities in the crag from overlying beds of superficial 

 detritus. 



On the conchological history of the crag, Mr. Clarke offers no re- 

 marks, partly because it did not fall within his object in writing the 

 paper, and partly because the data which he formerly collected have 

 been lost. 



Diluvium. — The diluvium of Suffolk may be divided into three 

 classes : 1 , clay ; 2, gravel ; and 3, erratic blocks. 



1. Clay. — This division covers a considerable portion of the 

 county, and extends into Norfolk, Cambridgeshire, and Essex, rising 

 to a considerable elevation in High Suffolk, and attaining near Cromer, 

 a thickness of 400 feet. A considerable portion of the clay, is of a 

 yellowish hue, but the greater part is blue ; and both varieties contain 

 chalk pebbles, sometimes disposed in layers but more commonly di- 

 spersed, a character by which the diluvial may be distinguished from 

 the London or plastic clay. It is difficult to determine the origin of 

 this argillaceous deposit ; but the author is inclined to think, that the 

 yellowish portion may have been derived from the plastic clays, and 

 the blue from the clays below the chalk. Fragments of coal have 

 been found in the diluvium at Lavenham, also fragments of mica slate 

 containing garnets and tourmaline. Specimens of a similar rock were 

 obtained by Mr. R. C. Taylor, at Cromer, with masses of granite, por- 

 phyry, trap, oolites, &c. At Ballingdon Hill near Sudbury, Mr. Brown 

 has procured thirty varieties of primary, secondary, and tertiary rocks. 

 Comparatively few flints occur in the clay. At Ickworth a beautiful 

 specimen of the Dudley Trilobite was obtained in making a drain : 

 and at various other localities, numerous species of tertiary and 

 secondary fossils abound. 



It is inferred that the clay contains cavities, as streams of noxious 

 air occasionally issue from fissures. 



2. Gravel. — The gravel is less generally diffused than the clay, and 

 is considered by Mr. Clarke, to have been partly deposited at a distinct 

 period. In some cases, it consists merely of unrolled flints, left in 

 situ by the dissolution of the chalk ; in others, large masses of flint 



