Frazer.] duU [March 19, 



seem to bear no relation to the latter. Besides, tlie supply of iron from 

 such minute crystals in the limestone would be insufficient to produce 

 the limonite beds. It seems much more probable that the source of the 

 supply of iron were the pyrite crystals of the slates which, once towering 

 high in the air, have been carried down by denudation and deposited in 

 the Atlantic. Even these slates which are not so situated as to permit 

 the percolation of water through them, exhibit a porous structure, the 

 pores being filled with brown ochreous limonite, and this occurs to a 

 considerable depth, and the slate merges by imperceptible degrees in 

 a direction normal to the plane of bedding, first into completely meta- 

 somatized pseudomorphs of limonite after pyrite (but still retaining 

 the form of the latter) ; then the same with a kernel of pyrite ; then the 

 pyrite itself, first with a shell and then with a rhere stain of ferric 

 hydrate ; and finally the same slates are revealed porphyritic from the 

 pyrite, but not at all decomposed. 



The question as to the source of the iron in these limonite beds, is this: 

 Does it come from the percolation and solution of its pyrite disseminated 

 through the more recent limestone, or does it come from the decomposed 

 pyrite in the slates of the same age ? For it will hardly be disputed, 

 that the main source of the supply consisted of pyrite, nor that the 

 decomposition of the slates into clays was the means of providing the 

 impermeable medium in which the iron solutions were caught and im- 

 prisoned. If the former hypothesis be the true one, we should ex- 

 pect to see an absence of limestone in the vicinity of the large deposits ; 

 for (granting for the moment that the limestone contains enough pyrites 

 to account for the entire deposit (a fact which at least admits of some 

 question), a percolation of water sufficient to oxidize the sulphur of these 

 pyrite crystals and carry away enough iron to produce the beds, would 

 entirely honey-comb and finally, both by solution and attrition, dissipate 

 the limestone bed. But in and near some of the largest limonite beds we 

 fin<l the limestone scarcely weathered, and in few cases, if any, is it ren- 

 dered ferruginous or even stained to any great degree by chalybeate 

 waters. Indeed, the absence of the familiar iron stain from the calcareous 

 member of this formation is so marked, that this point of difference from 

 the adjacent members of the series cannot fail to arrest attention. 



Again the uniformity of the occurrence of these limonite deposits on 

 the skirts of the basin and the lower edge of the elevated limestones and 

 their absence elsewhere, cannot but be the result of the law of their for- 

 mation. Were these deposits derived from the pyrite disseminated 

 through the limestone there would be no way of explaining the adher- 

 ence to the rule when the strata were highly inclined or vertical, except 

 by supposing that the ferruginous solution from the limestone found its 

 way across the decomposing slate beds in a direction perpendicular to 

 their planes of lamination — an hypothesis opposed to all experience. 

 But this would not account for the absence of iron oxide on the re- 

 maining edges of the limestone itself, for even if we could accept the 



