1873.] LUCAS — ORIGIN OP CLAY-IRONSTONE. 365 



Reverting to A. i, we find the beds so thin, and with so high a 

 percentage of iron, that it appears next to impossible that beds con- 

 taining it should be thrown down in layers in the midst of sediments 

 almost, and in some cases quite free from it, when we take into 

 consideration the fact that carbonate of iron, which is the salt con- 

 tained in most ferruginous springs, is rarely present in them in a 

 larger quantity than 1 grain per pint*. Of course I am aware 

 that thick deposits are formed in the open air at the mouths of such 

 springs ; but even if carbonate of iron exists in so high a proportion 

 anywhere in estuarine waters, the deposition of it in very thin beds 

 containing 30 to 40 per cent, of metallic iron, appears far less pro- 

 bable than that it should be generally distributed through a greater 

 thickness of mud in less quantity. 



«»For the formation of beds containing a percentage of iron cer- 

 tainly insignificant when compared with that of Clay-ironstones, 

 Prof. Ramsay supposes an inland basin to have been necessary ; 

 how much more, then, is it necessary that a bed containing 30 per 

 cent, of iron should have been formed, in circumscribed waters ? 



Again, concerning the varieties of Clay-ironstone which contain a 

 great excess of lime, and of the densely compact thin limestone 

 bands which occur in the millstone-grit series, there can be no 

 doubt that these beds have had a similar origin ; for carbonate 

 of lime can rarely be precipitated at the bottom of the sea by che- 

 mical action alone t • 



The formation of all varieties of Clay-ironstone lying in beds is ren- 

 dered intelligible by the supposition that they were formed in peaty or 

 non-peaty lagoons on the alluvial flats of the deltas of the Carboni- 

 ferous formations, according to the predominance or absence of 

 carbonaceous matter in the ironstone. Drained by evaporation and 

 percolation after the subsidence of floods, the stagnant pools occupying 

 these hollows would afford just the circumstances most favourable 

 for the deposition of iron from ferruginous, and lime from calcareous 

 waters, and from the chemical action described by Mr. Hunt. I 

 believe the cracks so often seen in beds of Clay-ironstone to have 

 been formed under these circumstances by the sun when the stag- 

 nant pools dried up, as they exactly resemble those formed in the 

 substance of ordinary clay under similar circumstances. The next 

 layer of mud transported into the bed filled them up, so that when 

 seen now they are found to be filled with shale, very often not in 

 the least ferruginous, and quite soft. 



This view explains the occurrence of layers of nodules equally well 

 if the surface of the lagoon presented slight undulations, or even in 

 some cases ripple-marks. 



It explains also why the beds of Clay-ironstone never run to a 

 greater thickness than about 2 feet, as the lagoons would be neces- 

 sarily very shallow, in fact nothing but slight hollows in the 

 alluvial flats, sometimes containing a film or bed of peat, sometimes 



* Miller's ' Elements of Chemistry,' pi. ii. p. G14, ed. 186-1. 

 t Student's Elements, p. 38. 



VOL. XXIX. — PART I. 2 1) 



