6 GENERAL ACCOUNT OF THE DEPOSITS. 



from the lower nodule seam in the slightly paler colour of its 

 phosphatic nodules, and in its scanty supply of carbonate of lime, 

 so that this seam remains constant as a loose pebble bed. 



The pebbles are the same as in the lower nodule bed. Amongst 

 them the cherts and Lydian stones are particularly interesting. 

 These vary greatly in colour, dark green to nearly black being 

 most abundant ; others are of lighter colours of yellowish and 

 chocolate hues. Frequently they are found with bands of colour, 

 or with projecting rings of more durable material. Many of the 

 chert pebbles are, to the naked eye at least, precisely like chalk 

 flints, both black and pale types being common 1 . 



It was in this bed that the workings for coprolites were princi- 

 pally carried on, and from it were obtained > a large proportion of 

 our Brachiopod fossils. 



(C. 4) The upper sand (C. 4) is similar to the lower seam 

 (C. 2) between the two nodule beds, already described. 



(C. 5) The bed of clay next in order has been referred to the 

 gault, but it is probably the representative of a bed of sandy clay 

 or clayey sand belonging to the Lower Greensand which was seen 

 in many of the Upware sections (e.g. the bed numbered (b. 4) in 

 the Spinney Abbey section) as was suggested by Prof. Bonney in 

 1875 (Cambridgeshire Geology, p. 64). This is the more likely 

 because phosphatic pebble beds (see bed Da in woodcut) occupy 

 characteristically the lines of chronological breaks, especially 

 throughout the cretaceous period. 



1 These chert and Lydian stone pebbles are very widely distributed in Neocomian 

 strata. They are found at Upware, Potton, Brickhill, and Farringdon, and in the 

 Bargate Pebble Beds, and they occur in precisely similar condition in the Hils 

 conglomerat of Schoeppenstedt in Brunswick. They are also abundant in the 

 Portlandian pebble bed of Bourton, near Swindon. Larger pebbles also occur, 

 mostly of quartz and quartzite. Some of the pebbles, especially those from Potton, 

 have yielded fossils, including shells, crinoids, <fec. of carboniferous age from the 

 chert ; Jurassic shells and echinoderms from other kinds of chert, and a number of 

 brachiopods (or this, spirifera, &c.) from a pale shaly slate, apparently of age of the 

 middle or Upper Bala group of Sedgwick. The details of these pebbles are given in 

 a paper read before the Cambridge Philosophical Society, May 3rd, 1880, and pub- 

 lished in the Geological Magazine, September, 1880. 



The majority of these pebbles I regard as having been derived from an ancient 

 Palaeozoic barrier axis which in the preceding, or Lower Neocomian period, 

 separated the Northern from the Southern Neocomian seas in Europe, but which 

 was in the time of the deposition of the Iron Sand series suffering rapid denudation 

 and destruction. 



