212 R. H. Rastall — Minerals of Lower Greensand. 



whether, on the other hand, there were from the "beginning gaps in 

 the succession owing to intervening land-areas. At all events, it is 

 certain that the Lower Greensand thinned out towards the south 

 against the high ground of the London Palaeozoic plateau, which was 

 first completely submerged in Gault times, as shown by the records 

 of numerous deep borings. The thickness of the Lower Greensand 

 within the district as above defined undergoes certain remarkable 

 and more or less regular variations, and the present author has 

 already put forward reasons for believing that this variation is due 

 to the existence in Lower Cretaceous times of a ridge of land running 

 in a general north-west to south-east direction, parallel to the 

 known strike of the Charnian rocks, and probably due to posthumous 

 movements of an ancient fold-line. 1 Repeated movements of this 

 axis have been clearly demonstrated in other instances by Professor 

 Kendall, 2 although he expressly disclaims any belief in its influence 

 on the Lower Greensand. On that point the present author ventures 

 to maintain his own opinion. It is at any rate a striking fact that 

 the strongly marked diminution in thickness of the Lower Greensand 

 takes place just over the district where it might be expected on this 

 supposition. In the west of Bedfordshire it is 250 feet thick, in 

 Norfolk about 150 feet, whereas at TJpware and Ely there are only 

 a very few feet, and it is possible that in places it is absent 

 altogether. These facts can only be accounted for by overlap 

 against a land-surface, apparently an isthmus dividing two seas, 

 which was gradually being submerged and was finally overflowed at 

 the beginning of TJpper Cretaceous times, since there is no doubt 

 that the outcrop of the Gault is continuous, although the thickness 

 of this formation diminishes greatly towards the north-east. 



The stratigraphy and palaeontology of the Lower Greensand 

 present many problems of great interest and difficulty* some of 

 them are still matters of discussion, as, for example, the true age 

 and origin of the so-called "derived" fossils, which are so common 

 in the pebble-beds of some districts. The present author is not 

 competent to deal with this and other cognate matters, nor with the 

 question of the exact correlation of the strata of the midland and 

 eastern counties with those seen elsewhere. The object of the 

 present paper is merely to set forth the results of a mineralogical 

 examination of the sands of certain areas in the hope that the facts 

 ascertained may assist in throwing light on some factors of a larger 

 problem, which must eventually be dealt with by others. 



1. Cakstone, Hunstanton. 

 The general character of the rock forming the lower part of the 

 cliff section at Hunstanton is so well known as scarcely to require 

 detailed description. It must suffice here to say that the finer beds 

 consist of a highly ferruginous sand, each grain being coated by 

 a pellicle of iron oxide, and the whole more or less cemented by the 

 same substance. Owing to this circumstance the preparation of the 

 material for minute investigation presents considerable difficulties. 



1 Rastall, Geology in the Field, 1909, pp. 140-4. 



2 Kendall, Report Coal Commission, 1905, pt. ix, p. 30. 



