26 



the form of minute globules which Dr. Hinde, the highest 

 English authority on such a subject, regards as being derived 

 from the spicules of sponges. 



The spicules in the nodules are seen to have passed, or perhaps 

 to be passing into the chalcedonic form of flint. In the higher beds 

 of the upper chalk in some parts of Yorkshire, and at Triming- 

 ham, nodules which have been evidently moulded to a great 

 extent on sponges, have been found in all stages of silicification 

 from the mass scarcely harder than the surrounding chalk and 

 containing but a trace of silica, to flints as black and as hard as 

 those in the chalk at Brighton. In this case the blackest and 

 hardest flint is in the interior of the nodules (Sollas " On the 

 Flint Nodules of the Trimingham Chalk"). 



In the oceans of the Cretaceous epoch there is little doubt 

 that sponges flourished to a far greater extent than at present. In 

 the greensand which lies at the base of the chalk there are beds 

 of chert, an impure variety of flint, and siliceous rock 20 or 

 30ft. in thickness and extending over many square miles. On 

 microscopic examination these beds are found to be largely com- 

 posed, or at any rate to have been derived from the spicules of 

 sponges (Hinde). One piece of malmstone from near Godalming 

 yielded on analysis no less than 72 per cent, of colloid silica (Way 

 and Paine). 



In the chert near Merstham in Surrey there are blueish-grey 

 nodules locally known as flints; in these the silica has aggregated 

 into denser masses than the rock which encloses it. These 

 nodules are disposed like the chalk flints in definite planes of 

 bedding. Dr. Hinde also speaks of sponge-beds, as they are 

 technically termed, in Westphalia, almost wholly composed of the 

 spicules and fibres of sponges, which extend to a great distance 

 and in places obtain the enormous thickness of 492ft. Now, 

 when we consider that the silica in the most siliceous sponge 

 known is not more then 12 per cent, of the total weight of a 

 flint the same size as the sponge, we may form some idea of the 

 extraordinary luxuriance of the sponges on the bed of this ocean, 

 and at the length of time that must have elapsed during the 

 accumulation of 500ft. of their debris. 



It is seen that there is no question as to the supply of silica 

 whatever its original source may have been. But the problem is 

 still to be solved why in some places the silica has segregated into 

 nodules and in others it is still disseminated through the rocks. 

 It is no doubt at bottom a chemical question. Wherever the 

 calcareous strata are pure carbonate of lime there the flints are 



