380 W. H. HUDLESTON ON THE CHEMICAL 



The very -small amount of phosphoric acid found by Dr. Daubeny 

 in the apparently unfossiliferous portions of these rocks in North 

 Wales appears to have been in a remarkably soluble form ; for as on 

 the one hand the dolomite of Roche Abbey, which is probably crys- 

 talline, yielded during the first year no more phosphate in the 

 barley-ash than did the Cambrian slate of Dolgelly, still in the 

 second year, whilst this latter was entirely exhausted, the dolomite 

 yielded about three times as much phosphoric acid in the barley-ash 

 as it had done in the first year. This points to the probability of 

 the phosphate in the Cambrian rocks under examination having been 

 in a readily soluble form ; and this, indeed, is a conclusion towards 

 which the analysis of Mr. Hicks's specimens of Menevian flags also 

 draws us. Subcrystalline limestones and dolomites may, then, hold 

 their phosphate in a more crystalline, and therefore less soluble, 

 form — not unlikely as chlor-apatite. Apatite generally is stated by 

 Bischof to be about three hundred times less soluble in carbonated 

 water than some varieties of non-crystallized calcic phosphate. We 

 may therefore legitimately infer that the phosphate of the Cambrian 

 rocks, as far as we have analyses of these, does not exist in a crystal- 

 line form. 



This brings us to a consideration of the source of the phosphate in 

 the Menevian beds — and incidentally to that much-vexed question, 

 the origin of phosphatic accumulations in the successive formations. 

 In the Trilobite-flags this must be, as suggested by Mr. Hicks, largely 

 due to Crustacean remains, both flesh and shell containing, as we 

 have seen, a considerable quantity of phosphoric acid. It is of course 

 impossible to say with absolute certainty what proportion of phos- 

 phoric acid the great Trilobite of St. David's contained in his shell ; 

 and, indeed, we should require a large series of analyses of several 

 orders of existing Crustacea before we should be in a position to decide 

 as to whether the percentage differs materially in the several orders. 

 But we may fairly say that, having once established the fact of 

 Crustacea being rich in phosphoric acid, the more bulky forms would 

 yield a richer deposit, because, even supposing their percentage 

 to be the same as that of the smaller forms, their buried bodies 

 would displace a greater amount of sediment, and on the putrefac- 

 tion and ultimate disappearance of their organic constituents, their 

 mineral residuum or ash, consisting chiefly of phosphates and 

 sulphides, would bear a greater ratio to the ordinary sediment than 

 in the case of a smaller creature taking up less space. At the 

 same time these Crustacea would derive their phosphoric acid, as 

 well as all other constituents, from their food ; and therefore such 

 animals can, for geological purposes, be only viewed as collectors in 

 a well-concentrated form of substances existing more diftusedly in 

 other organisms. 



The primary origin of phosphoric acid, if it is necessary to go so 

 far back, must naturally be sought in minerals which are decom- 

 posed by aqueous and atmospheric solvents, whereby their phosphates 

 are rendered available to plants and, through these, transferred to 

 animals. But the commencement of this cycle of events is to be read 



