197 



aatilis belongs to die genus Crioceras, whilst the Hamites intermedins 

 and beanii are both species of Ancyloceras, a genus which is cha- 

 racteristic of the lowest beds of the cretaceous series. To these 

 we may add Spatangus retusus ; so that it becomes very probable 

 that the Speeton clay may also be the equivalent of the lowest ar- 

 gillaceous Neocomian strata of the chalk series of the south-east of 

 England. 



The lower greensand of the south-east of England, as round the 

 Weald and in the Isle of Wight, rests on the Neocomian group ; and 

 it is when so placed that it attains its greatest thickness : it does not 

 extend westward, but thins away beneath the great expanse of chalk 

 of the counties of Hants and Wilts. It reappears a little south of 

 Tetswoi'th, and increases in thickness in its extension to the north- 

 east through Cambridgeshire. In this part of its course it out- 

 spreads the freshwater deposits, and rests unconformably upon sub- 

 divisions of the oolitic series. 



The gault, though inferior in thickness to some of the other 

 groups, is the best horizontal line by which they may be severally 

 arranged, both on account of the well-defined lines by which it is 

 separated from the groups both above and below it, and also from 

 the peculiar fossils it contains. It occurs round the western district 

 and in the Isle of Wight, immediately above the ferruginous sands. It 

 reappears from beneath the chalk at its escarpment at Shaftsbury, and 

 in the Vale of Wardour, where it has been fully traced out by Dr. Fit- 

 ton, but cannot be identified beyond there. In the Vale of Wardour, 

 instead of following upon the lower greensand, it occurs upon the 

 marginal beds of the Wealden as well as upon Portland and Kimme- 

 ridge strata. As was noticed with respect to the lower greensand, the 

 gault also seems wanting along a considerable interval, but reappears 

 about Tetsworth, and acquires its greatest thickness in Cambridge- 

 shire. 



Reliance upon the mineral character of the gault, or rather of one 

 particular portion of it, viz. the argillaceous, has caused it to be 

 overlooked in its extension westward, over the counties of Dorset 

 and Devon. The lower beds of the sands which cap the hills from 

 Lyme to Sidmouth belong to the gault, and the shingle bed, which 

 the author has noticed as occurring in this portion of the gault series, 

 and which may easily be seen in Salcome Bilinear Sidmouth, marks 

 its upper limit. The pebbles he believes have been derived from the 

 Portland sands of Dr. Fitton. 



The interval along which neither the gault nor lower greensand 

 are to be found is not owing, as is sometimes the case, to the sub- 

 stitution of one set of beds for another, but will be found to corre- 

 spond exactly with the rise of the older strata from Frome westwards, 

 and which elevation is of earlier date than any portion of the creta- 

 ceous series. 



Everywhere intermediate between the chalk and the gault is the 

 complex group (the upper greensand) noticed in the preceding page. 

 In its extension west, this group becomes wholly siliceous, and forms 



