62 PEOCEEDOTGS OF THE GEOLOGICAL SOCIETY. [Dec. 4, 



I believe this to be a much more probable account of tbe deriva- 

 tion of these nodules than that which attributes them to denudation 

 of the Gault. It is true that nodules of a somewhat similar cha- 

 racter are found below in the Gault ; and I have no doubt that their 

 origin is likewise organic ; but they are for the most part much smaller 

 —dwarfed, as if the muddy waters of the Gault sea did not suit 

 them ; they are also very sparsely scattered in the clay. 



In the history of these nodules we have an instance of what is 

 taught us by many similar facts in geology, namely the rapidity with 

 which petrification has taken place — so that we find fossils, after 

 having been completely mineralized, redeposited in another stratum 

 of what we are in the habit of calling the same deposit. 



When we compare the Lower Cretaceous beds of this district with 

 those of the west and south of England, we are struck by the absence 

 of that great arenaceous deposit, the Upper Greensand, while the 

 lower beds of the Chalk in both areas are extremely alike. We find 

 in Hampshire and in Dorsetshire a thin band very similar to our 

 phosphatic bed, which, like it, passes upwards into a Chalk-marl 

 with glauconite grains. But the point of difference is, that here the 

 nodule-bed rests upon the Gault, whereas there we have the great 

 arenaceous deposit intervening. In both districts this thin band 

 appears to represent a long period. It is probable that it is the 

 washed remnant of a glauconitic marl-deposit in both districts. 



We have, then, to account for the absence of the arenaceous green- 

 sand in the Eastern Counties. It is probably due to the ridge of old 

 rocks beneath the London area, which shut off the early Cretaceous 

 sea to the north 01 it from those south-western lands which yielded 

 the sanely spoils. 



And if it be not going beyond the scope of this paper, I may men- 

 tion that I suspect a similar cause to have produced the marked 

 change between the Lower Cretaceous rocks in Cambridgeshire and the 

 corresponding beds in Norfolk and Lincolnshire. 



A glance at a geological map of England will show the outcrop of 

 the secondary rocks thereabouts making a semicircular sweep, having 

 the old rocks of Charnwood forest in its centre. We have also a Pa- 

 laeozoic slaty rock within about a thousand feet of the surface at 

 Harwich. 



If those two points be joined by a line curving slightly northward, 

 just parallel to the axis of the Weald, such fine will represent the 

 direction of the slight elevation to which the curvature of the outcrop 

 of the Secondary rocks is due, and will pass through the area where the 

 character of the Lower Cretaceous rocks changes from nodule-bed and 

 Gault into Eed Chalk. A second Palasozoic ridge following that di- 

 rection would account for this change * ; and we might look to the 

 Trias, which on every side of Charnwood abuts upon the Cambrian 

 rocks, to have furnished the ochreous deposit giving rise to the B,ed 

 Chalk. 



* Several of the erratic pebbles of tbe phosphate-bed may be matched by 

 rocks from Charnwood. In Mr. Jesson's collection I hare seen pebbles of a 

 very peculiar, greenish, flinty slate which occurs in situ near Whitwick. 



