12 GENERAL A.CCOUNT OF THE DEPOSITS. 



(3) The coprolite bed; a yellowish sand sometimes hardened by 

 iron oxide, in which the Phosphatic Nodules or 'Regulars' 

 (as the workmen style them) lie. Fossils. About 1 ft. 2 in. 

 (2) A rock bed or 'carstone,' ironstained, 5 in. 

 (1) Bed, white and green sands; proved by a well-boring to be 



20ft. 



Both the "Regulars" of the true coprolite bed and the "Militia- 

 men" of the gault are pressed into the service of manure manu- 

 facture, and are mixed up together in the heaps of nodules. The 

 'Militiamen' are of the usual gault type, but the 'Regulars' are 

 different from any that we have yet noticed. These are rather 

 large and rounded nodules, but unworn ; and we may distinguish 

 two kinds amongst them (1) those of a dirty white colour, and (2) 

 the dark green ones. Their surface is not smooth as with the much 

 eroded derived nodules of other places, but rough and scabrous 

 with projecting grains of quartz, &c., as though sand had been 

 sprinkled upon a viscid hardening mass. This is especially the 

 case with the pale forms. 



Breaking a nodule to expose a fresh-fractured surface we see it 

 is a quartzose, and even pebbly sandstone of varying coarseness, the 

 grains cemented together by a dense matrix of phosphate of lime 

 and iron oxide. The dark green nodules are of an older date of 

 origin for they are frequently included within the paler ones. In 

 internal structure the two types are similar, but the green ones 

 are perhaps usually more compact, slightly darker in colour, and 

 apparently richer in phosphate than the paler ones. The fossils of 

 the Downham coprolite bed are principally in the state of internal 

 casts in a dark variety of phosphate, more like those of the Cam- 

 bridge greensand and the mammillaris-zone nodules of the south 

 of England and the Ardennes 1 . 



Origin of the ' coprolites.' It is a very general rule that phos- 

 phatic pebble beds are found along lines of ancient erosion and 

 unconformity, the nodules having been derived from the destruction 

 of older rocks. Thus the 'coprolites' of the Suffolk Crag are de- 

 rived from the London Clay and those of the Cambridge Greensand 

 to a great extent from the gault. Just similarly the nodules of 

 Upware and Brickhill have been derived, for the most part, from 



1 The Gault nodules of the Perte du Rhone and the Bala Phosphates belong to 

 the contemporaneous class. 



