Flints of the Upper or White Chalk. 195 



this position in relation to the calcareous mud, upon which 

 they may be said to float so as to form an intermediate stratum 

 between them and the superincumbent water) will become, 

 if not supersaturated with silica, at all events so highly 

 charged with it in a now colloid state more and more closely 

 approaching coagulation, as eventually to asphyxiate, so to 

 speak, the very organisms which have produced them. 



'' If we turn to the less prominent, because negative, con- 

 ditions that prevail at the sea-bed, we shall perceive that they 

 are of a kind specially favourable for securing uniformity of 

 results, both as regards the time occupied in their completion 

 and the nature of the changes which are effected by them. 

 Thus we know that the abyssal waters closely bordering on 

 the sea-bed itself are, in the majority of cases, in a state so 

 nearly approaching perfect quiescence, that no current of 

 sufficient energy exists to divert fi'om their downward course 

 particles of matter so light and feathery as to have taken 

 probably many weeks, if not mouths, to sink down from the 

 surface of the sea to their tinal resting-place at the bottom. 

 On the other hand, there is nothing as yet known that could 

 lead to the inference that the periods required for the depo- 

 sition and consolidation of each succeeding stratum of chalk, 

 and its accompanying stratum of flints, bear any proportion to 

 those gradual and more rarely recurring secular changes in 

 the direction of the great oceanic currents which (to repeat 

 Sir Charles Lyell's words) favour at one time in the same 

 area a supply of calcareous, and at another of siliceous matter ; 

 whilst, as a natural consequence, the prevailing uniformity of 

 the physical conditions must inevitably engender a corre- 

 sponding uniformity and simultaneousness in the development, 

 growth, and final death and decay of the various lower forms 

 of life that are under its inflnence. If this be true, we might 

 expect that over large areas of the calcareous sea-bed a very 

 preponderating number of the sponges would, almost simul- 

 taneously, spring into existence from the germs or gemmules 

 left by a preceding generation, and as simultaneously multiply 

 and die, to be succeeded in turn by another generation, and 

 so on. We are thus furnished with an auxiliary, though (as 

 1 shall presently show) by no means the most important, 

 factor in determining the simultaneous production of the flint 

 nodules and sheets over extended horizontal areas." 



" The stratification of the flints is due to the fact, already 

 touched upon in a previous page, that nearly the whole of the 

 silex derived from the Sponges on the one hand, and the 

 continual subsidence of minute dead siliceous organisms on 

 the other, is retained in the general protoplasmic layer, which 



