THE CAMBETDGE GAULT AND GREENSAND. 267 



would have done near Cambridge, where, he told me, he had seen 

 the coprolites accumulated to a depth of 2 feet in such places. 



At the time I was inclined to think the undulations were here pro- 

 duced by small local rolls in the beds themselves, but have since 

 thought it possible that a slight erosion of the Gault below the 

 nodules may have caused them ; and I am rather disposed to look upon 

 every band of black phosphatic nodules as representing the washings 

 of some bed of clay or other material in which they were originally 

 formed. This, however, raises the question of the actual formation 

 of phosphatic nodules, into which I do not now desire to enter. My 

 object is to trace the Cambridge coprolites to the " gisement " which 

 they occupied before they were transferred to their present position ; 

 and I think enough has already been said to make it probable that 

 this layer in the Gault was the home of a great many of them. 



The same bed is worked again at Cheddington, three miles to the 

 N.E., and near the London and North-western Eailway. Here it is 

 not quite so sandy as at Puttenham, the green grains occurring more 

 in patches among the nodules. Still the quantity is sufficient to form 

 a greensand, were the bed subjected to the action of running water 

 or abrading currents. 



Now this very operation is actually performed by the process 

 of washing the coprolites ; and one result of this is to leave a small 

 heap of pure greensand near the washing-mill, of exactly the same 

 appearance as the heaps in a similar position throughout Cambridge- 

 shire. 



Thus these sand-heaps clearly prove that, if the bed were to be 

 eroded in a more natural manner, the result would be a clayey 

 greensand containing numerous waterworn coprolite nodules, such 

 as we find in the so-called Cambridge Greensand. 



I lay much stress on this fact, because the occurrence of glauconite 

 grains in the light- coloured indurated matrix which often fills the 

 interior of the Cambridge nodules has been a difficulty to some 

 geologists, preventing their acceptance of the Gault origin of the 

 coprolites. 



Thus Mr. Fisher speaks in the following terms * : — " The indurated 

 matrix which fills the axes of the cylindrical nodules was in all 

 probability introduced while the fossils were in their original gise- 

 ment. Hence we gain a clue to the derivation of the fossils of the 

 thin crowded layer of the so-called Upper Greensand of Cambridge. 

 They seem to have been washed out of a calcareous marl similar in 

 character to the marl which lies above it. In short, the nodule-bed is 

 a condensation of the ' Chalk-marl with glauconite grains.' " He pro- 

 ceeds to say that he believes this to be a much more probable account 

 of the derivation of these nodules than that which attributes them 

 to the denudation of the Gault, since these are for the most part 

 smaller, " dwarfed, as if the muddy waters of the Gault sea did not 

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



This is quite true of the Cambridge Gault ; but if it has suffered 

 denudation, the beds which really furnished the coprolites would no 

 * " On Phosphatic Nodules," Quart. Journ. Geol. Soc. vol. xxix. p. 61. 



