80 UE. WALLICH ON THE PHYSICAL 



not extend beyond half an inch, or an inch, below the surface, and 

 it is then replaced by a stratum of the utmost tenacity. This marks 

 the limit of the amorphous non-mineral particles, and the point at 

 which the consolidation of the heavier atoms begins. It appears 

 probable that the living Olohigerince are altogether confined to this 

 superficial stratum." (Op. cit. pp. 138, 139.) 



Of the source of the subsidiary protoplasmic layer but little 

 need be said in this place beyond pointing out that a very consider- 

 able portion of it is, in all likelihood, derived from the Glohigennce, 

 which constitute, as a rule, from 70 to 85 per cent., but in certain 

 rare instances as much as 95 per cent, of the entire surface stratum 

 of the calcareous deposit itself. The quantity furnished by the 

 Glohigerince, irrespectively of the other Hhizopodal families associated 

 with them in varying, but always in comparatively insignificant 

 proportions, must therefore be very considerable. 



It has been shown how vast must be the supply of organic silica 

 derived from the deep-sea Sponges. It is therefore quite intelligible 

 why, so long as this predominant source both of protoplasm and 

 silica remained unrecognized, undue weight should have been 

 attached, by some writers, to the quantity of the latter material pro- 

 vided by the Polycystina, Dlatomacece, and the still less numerous forms 

 of siliceous-shelled Protozoa, and by others to the quantity brought in 

 solution by the waters of the ocean from land sources. That the 

 siliceous remains of the organisms referred to do occasionally occur 

 in sufficient abundance to satisfy the wants of the microscopist, is in- 

 disputable. The silica of the flints cannot, however, be said to depend, 

 in any great degree, on these microscopic forms, as any one familiar 

 with the extent and structure of the siliceous skeletons and spicules 

 of the sponges will admit. But, in addition to this, I can state with 

 confidence that in no part of the North Atlantic, where the Globi- 

 gerine mud prevails (the mud declared to be a continuation of the 

 old chalk), do the remains of Polycystina or Dlatomacece constitute, at 

 the most, more than from 4 to 6 per cent, of the entire deposit ; and 

 in no instance do they occur, either separately or collectively, in 

 such numbers as to interfere with or " mask " the typical character 

 of the calcareous mud ; nor, if we look at the matter only in its 

 relation to the flints, can we come to any other conclusion than 

 that their production, though of course increased to a very trifling 

 extent by accessions of silica from the whole of the subsidiary 

 sources alluded to, would not have been appreciably influenced 

 had the Polycystina and Dlatomacece been altogether wanting*. 



"^ I have to observe in this place that it has been too much the custom in 

 estimating the percentage of the various materials present in.the deep-sea depo- 

 sits, whether organic or inorganic, to base inferences on what is observed under 

 the microscope, quite forgetting that all mounted objects such as Diatoms or 

 Coccoliths are not even visible except as mere specks, unless seen under the high(sr 

 powers of the instrument. To identify the form of a Coccolith, for example, 

 under a lower power than ^ objective, is almost impossible ; but in order to see 

 it at all the whole of the heavier particles must be entirely removed, and the 

 lightest particles, amongst which are the Coccoliths and Diatoms, must be 

 separated by a process of elutriation. To deduce percentages from elutriated 

 residues is obviously, therefore, misleading in the highest degree. 



