E. Wethered—IIcematite Deposits. 525 



Coal-measures and Millstone-gvi't, by the percolation of water 

 through the strata, yet I doubt not that the main portion was derived 

 from the Trias. I do not, however, agree with Mr. Etheridge that 

 the infilling took place "during the progress of the denudation of 

 the Palfeozoic rock," but that it was occasioned by percolation of 

 water through the Triassic strata after consolidation. This water 

 on arriving at the Carboniferous Limestone would flow down the 

 fissures and joints wherever these occurred ; but where the strata 

 was not fissured, a comparatively small volume would filter through 

 the actual rock — on account of its being but slightly porous. Ac- 

 cording to observations which I have recently made on the porosity 

 of rocks, I find that a cubic foot of Carboniferous Limestone is 

 capable of absorbing 0-043 of a gallon of water, or 3^ million 

 gallons to a square mile 3 feet thick. I take it, then, that it is owing 

 to this fact that we generally find htematite where the Magnesian 

 Conglomerate rests upon the Carboniferous Limestone. It is much 

 the same with the Millstone-grit of the Bristol Coal-field. A cubic 

 foot of this rock absorbs 0-081 gallon of water, or 6 million gallons 

 to a square mile 3 feet thick. In South Wales, however, the same 

 formation becomes more porous, and is capable of absorbing 0-290 

 gallons to a cubic foot of rock, or 23 million gallons to a square 

 mile 3 feet thick. In the Forest of Dean the grit absorbs 0-854 of 

 a gallon of water to a cubic foot, equal to 71 million gallons to 

 a square mile 3 feet thick. Now, it is worthy of remark, that there 

 are no workable deposits of haematite in the Carboniferous Lime- 

 stone around Bristol, where the Millstone-grit is very impervious to 

 water, but they are extensive in South Wales and in the Forest of 

 Dean, where it is more pervious to water. 



To sum up my conclusions on the subject of this jDapei', I regard 

 the so-called pockets of haematite in the interior of the Limestone 

 as caverns into which water has percolated charged with the car- 

 bonates of lime and iron. On the excess of carbonic acid being 

 given ofi", and on complete saturation taking place, carbonate of lime 

 first separated in the crystalline and crystallized state ; and subse- 

 quently, when oxidation was possible, peroxide of iron was thrown 

 down. 



As to the source from which the iron was derived, I think it 

 possible that some portion may have come from the Coal-measures 

 and Millstone-grit prior to denudation by the Triassic sea, but that 

 the greater portion was derived from the Trias rocks by the percola- 

 tion of water containing carbonic acid. I may just add that I am 

 informed that in the Carth Mine the deep ore is of a superior quality 

 to that met with higher up, and that it resembles more the Spanish 

 mineral. 



In conclusion, I beg to return my thanks to Mr. Monks, jun., of 

 the Easton Collieries near Bristol, to Messrs. Watkins and Howell, 

 managers of the Carth Mine, and to Mr. Henry Jeflferis, of Pentyrch, 

 near Cardiff, for permission to visit the mine. 



