»2 DE. WALLICH ON THD THYSICAL 



arrive when the protoplasmic masses (which, owing to their inferior 

 specific gravity, always occupy this position in relation to the cal- 

 careous mud upon which they may he 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 ap- 

 proaching coagulation, as eventually to asphyxiate — so to speak — 

 the very organisms which have produced them. 



If we turn to the less prominent, because negative, conditions 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 suficient energy exists to divert from their down- 

 ward course particles of matter so light and feathery as to have 

 taken probably many weeks, if not months, to sink down from the 

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

 other hand, there is nothing as yet known that could lead to the in- 

 ference that the periods required for the deposition and consolidation 

 of each succeeding stratum of chalk, and its accompanying stratum 

 of flints, bear any proportion to those gradual and more rarely re- 

 curring 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 corresponding uni- 

 formity and simultaneousness in the development, growth, and final 

 death and decay of the various lower forms of life that are under 

 its influence. 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 simultaneously^ 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 I 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. 



Although the analyses now about to be quoted are somewhat out 

 of place in this portion of my paper, I introduce them here as a re- 

 quisite preliminary to some observations on colloid phenomena which 

 follow, it being of importance that their bearing on these should bo 

 clearly understood. 



The first of these analyses to which I have to direct attention 

 is one, by the late Mr. David Forbes, of a sample of '^ Atlantic 

 ooze," obtained from a depth of 1443 fathoms to the south-eastward 

 of the Rockall shoal, off the north-west co^st of Ireland, during the 

 cruise of the 'Porcupine' in 1869. " A complete analysis of this 



