18 Rev. J. F. Blake — Form of Sedimentary Deposits. 



to say in what proportions each zone of depth contributes to the 

 production of limestone by the organisms inhabiting it, because 

 we cannot state the average number of the individuals in a given 

 area which make calcareous hard parts, nor the weight of each hard 

 part produced ; but we can get some idea of the probable position 

 of the maximum (on the principle that the more individuals there 

 are struggling for existence, the more species will result) by 

 availing ourselves of the results of the Challenger Expedition con- 

 cerning the number of species met with in various zones of depth, 

 as shown in the following table : — 



Table shoiving the number of species of Calciferoiis Organisms in various zones of 



depth in the sea. 



It is here seen that 82-7 per cent, of the calciferous species occur 

 in the uppermost 500 fathoms, and from the rapid rate of increase 

 probably 80 per cent, in the uppermost 300 fathoms. Add to 

 this that calcareous alg£e abound with corals, and also without them, 

 in the highest zone, and that, according to Pourtales, a bank made 

 of Globigerinse occurs in 100 fathom water off the coast of Florida, 

 and we may feel confident that (possibly with a few exceptions) 

 limestones containing an abundance of any of the above-named 

 organisms cannot have been formed at a depth greater than 300 

 or 400 fathoms. 



Thus the first two conditions for the flourishing of calciferous 

 organisms are found to practically prevail, and the third must 

 be satisfied in some other way than by remoteness from the shore, 

 probably by the absence of mud-bearing streams in their neigh- 

 bourhood. 



Furthermore, it may be noted that the fauna of the deep sea is 

 remarkable for its constancy, but the faunas of shallow water and 

 of limestones are remarkable for their variation from spot to spot. 

 Limestones are also largely detrital, not merely made of comminuted 

 parts, but of impalpable calcareous mud — as in the Lias, the Chalk, 

 and most marbles — and this detrition must have occurred in the zone 

 of detrition, usually limited, as noted above, by the 100 fathom line. 



(To be continued.) 



