62 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 4,
I believe this to be a much more probable account of the derivation of these nodules than that which attributes them to denudation
of the Gault. It is true that nodules of a somewhat similar character 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 greensand 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 of 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 mention 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 Palaeozoic 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 Red Chalk. A second Palaeozoic ridge following that direction 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 Red Chalk.
- Several of the erratic pebbles of the phosphate-bed may be matched by
rocks from Charnwood. In Mr. Jesson's collection I have seen pebbles of a very peculiar, greenish, flinty slate which occurs in situ near Whitwick.
