180 Dr. Wallich on the Origin of the 



cranny, have become converted into one stupendous pseudo- 

 morphic mass of compact silicified limestone. 



It must not be imagined, however, that the views now so 

 confidently advocated are based on mere assumption unsup- 

 ported by a fair amount of relevant evidence — as relevant 

 perhaps as any evidence can be that relates to natural opera- 

 tions tliat may in times past have been, or may even now be, 

 carried on at abyssal depths in the ocean *. I have from time 

 to time, during a long-continued study of the flint question, 

 seen specimens of limestone thickly studded with fossil dia- 

 toms, not one of which, even when examined under a micro- 

 scopic power amply sufficient to exhibit any loss of substance 

 or form, exhibited the least trace of having undergone solu- 

 tion ; the calcareous matrix of the limestone, crystalline and 

 apparently deposited from solution, enclosed the diatom-valves, 

 which remained as distinct from each other, although in the 

 closest mechanical contact, as it was possible for them to be. 

 It is, doubtless, both possible and probable that some of the 

 more delicate of these structures may have undergone com- 

 plete solution ; for, as pointed out by me nearly twenty years 

 ago, the Acanthoinetrce, a remarkable and very beautiful group 

 of siliceous organisms inhabiting only the surface-waters of 

 the open ocean, often in immense numbers, are never found in 

 recent or fossil oceanic deposits. This I showed to be the 

 result of the unusually large admixture of basal protoplasm 

 with silica, of which their spines are composed, and which 

 imparts to them a very distinct optical character, causing them, 

 in virtue of this excess of colloidal matter, invariably to dis- 

 solve away in sea-water before their remains can sink down 

 to the bottom. Some diatoms likewise present this character ; 

 and accordingly these may, if they formerly existed, have 

 vanished from the limestone under notice. But, as already 

 stated, the whole of those still visible remain perfectly intact, 

 and, Avhen seen in delicately cut sections, retain their characters 

 so perfectly as to enable their marine origin to be positively 

 determined. 



As bearing directly on this question, I will here quote 

 from my ' North- Atlantic Sea-bed,' published in 1862, with 

 a view to show that even at that early date I had given the 

 subject some careful consideration, though all ray conclu- 



• It is almost needless to point out that, in all questions relating to the 

 conditions and changes taking place at the bottom of the ocean, assump- 

 tion and hypothesis must, for many a day to come, occupy the place of 

 demonstrated facts. Hypotheses are the advanced guard of knowledge, 

 and, if properly equipped and cautiously sent forth, minimize the risks of 

 fallacy when exploring an unknown region in science. 



