200 Dr. Wallich on the Oriqin of the 



interstratified flint-lajers, and showing neither symptoms of 

 exhaustion nor diminution of size at any part of their upward 

 range. 



The inference I would draw from these facts is that, 

 whereas the living portion of each individual sponge was 

 restricted to one plane, and that plane was determined by, 

 and therefore followed, each rise in the level of the surrounding 

 deposit, the growth being due to simple repetitive divisions of 

 parts, and not to a process of reproduction, until the maximum 

 height and perhaps maximum age attainable by each indivi- 

 dual had been arrived at, the death of the parent Titan was 

 synchronous with and perhaps dependent on the intervention 

 of a true reproductive process, whereby a successor was pro- 

 duced, who was destined to pass through a similar cycle of 

 existence. We may assume also that the enormous size of 

 each individual, as compared with the other sponges and 

 forms of animal life that passed their lives on the same sea- 

 bed, would enable it to rear its head high enough above the 

 general level, when occasion demanded, to enable it to con- 

 tinue its existence uninterruptedly while the organisms around 

 were perishing. 



The stratification of the flints in layers of nodules and 

 tabular masses may^ I conceive, be similarly accounted for. 

 Starting with the facts that the calcareous areas of the ocean 

 (wliich are the representatives of those in which the ancient 

 chalk was deposited) consist of vast expanses of this deposit, 

 interrupted only by sponge-fields and sponge-beds (the one 

 living and flourishing in the intervals from which it had 

 either gradually expelled or yielded up its ground to the 

 other) , what must have occurred, and be still occurring, over 

 the calcareous sea-bed ? As the sponges encroached (in virtue 

 of their undoubtedly more rapid growth *) on the domains 

 of the Foraminifera, the latter would, here and there, be 

 overwhelmed by the protoplasmic masses and simply asphyx- 

 iated. The sponges would, in turn, encroach on each other, 



* Prof. Martin Duncan says, with reference to the slow rate at which 

 deep-sea deposits are formed : — " With reference to the great thickness 

 of deep-sea deposits, I have satisfied myself, from late researches, that 

 the rate of deposition is exceedingly slow. Thus an electric cable was 

 laid down in the Globi(jerina-oozQ region ; and six years after a consider- 

 able coral-growth had taken place on it. Some of the liviug calices were 

 close above the cables ; and therefore the deposit had been intiuitesimal in 

 that time. Again, there are slow-growing Echinoderms, Corals, and Spon- 

 gida in place in many chalk series ; and it is evident that the foramini- 

 feral and sedimentary deposit was infinitely slower than their growth " 

 (Anniversary Address Geol. Soc. Loudon, 1877, by Prof. Martin Duncan, 

 M.B., F.K.S., p. 44j. 



