18 FOSSIL MEDUSA. 



from the sea water without the intervention of any organic agency. In 

 answer to this suggestion, and also to show the drift of opinion in relation 

 to the origin of the silica forming the flints of the Cretaceous, I will quote 

 the opinions and conclusions of several authors. 



In a contribution to the physical history of the Cretaceous flints, 1 Dr. 

 Wallich arrived at the following conclusions: 



1. That the silica of the flints is derived mainly from the sponge beds and sponge 

 fields which exist in immense profusion over the areas occupied by the globigerine or 

 calcareous " ooze." 2. That the deep-sea sponges, with their environment of proto- 

 plasmic matter, constitute by far the most important and essential factors in the pro- 

 duction and stratification of the flints. 3. That whereas nearly the whole of the 

 carbonate of lime, derived partly from Forauiinifera and other organisms that have 

 lived and died at the bottom, and partly from such as have subsided to the bottom 

 only after death, goes to build up the calcareous stratum, nearly the whole of the 

 silica, whether derived from the deep-sea sponges or from surface Protozoa, goes to 

 form the flints. -1. That the sponges are the only really important contributors to the 

 flint formation that live and die at the sea bed. 5. That the flints are just as much an 

 organic product as the Chalk itself. 6. That the stratification of the flint is the 

 immediate result of all sessile protozoan life being confined to the superficial layer of 

 the muddy deposits. 7. That the substance which received the name of " Bathyb- 

 ius," and was declared to be an independent living Moneron is in reality sponge 

 protoplasm. 8. That no valid lithological distinction exists between the chalk and 

 the calcareous mud of the Atlantic; and pro tan to, therefore, the calcareous mud may 

 be and in all probability is a continuation of the Chalk formation. 



Dr. Sorby, in discussing these conclusions of Wallich's, stated 2 that he — 



had formerly studied this subject and come to the conclusion that, though deep-sea 

 mud differs from chalk in many important particulars, yet still it was sufficiently 

 related to warrant a comparison. Since the remains of siliceous organisms are absent 

 from the chalk, but flint present, whilst in the deep-sea mud siliceous organisms are 

 abundant and flints absent, probably the material of the flints had been to a greater 

 or less extent derived from these organisms. Much, however, remains to be learned. 



Prof. W. J. Sollas, in his article on the flint nodules of the Trimming- 

 ham chalk, 3 in commenting on this argument, considers that it is not an 

 analogy, but is in reality nothing less than a statement of fact. He then 

 proceeds to prove that the Trimmingham flints have not only sponge spicules 

 intimately associated with them in great numbers, but that the spicules 

 afford clear proof of the previous existence of a great mass of spicules of 



1 Quart. Jour. Geol. Soc. London, Vol. XXXVI, 1880, p. 90. 



2 Loc. cit., p. 91. 



3 Aunals Mag. Nat. Hist., 5th series, Vol. VI, 1860, p. 438. 



