524 Reports and Proceedings — British Association — 



course any excavation in the rock becomes a cavern, hut the pockets 

 are natural caverns ; all the miner does is to work out the ore. I also 

 took occasion to ascertain what were the signs which indicated the 

 approach to ore, and I was informed that they were as follows : — 



1. Joints appear in the limestone through which water percolates. 



2. An ordinary cavern appears, termed by the Welsh miners a 

 " Locus," the sides of which are often covered by crystalline and 

 crystallized carbonate of lime. 



3. Traces of ore appear in the "Locus." 



The fact that the first indications of ore are cracks in the rock 

 down which water percolates certainly points to the inference, that 

 it was by a like percolation that the heematite has been brought into 

 its present position. 



The next point for consideration is ; from whence was the iron 

 derived. The highly feri-uginous condition of the Carboniferous 

 strata is well understood, and the fire-clays^ indicate that large 

 quantities of iron have been reduced to the soluble state and 

 removed by water. But as to whether it is this iron which has 

 been deposited in the limestone is a matter for consideration. It is 

 questionable whether sufiScient time could have elapsed for the 

 formation of the caves to admit of the supposition that the iron had 

 been derived from these rocks prior to denudation by the Trias sea. 

 We must, however, remember, that the denudation extended over a 

 great lapse of time, during which much of the limestone now not 

 covered by the Coal-measures or Millstone-grit was overlain by these 

 formations. 



Mr. Etheridge considers the Carboniferous haematite deposits in 

 the West of England to have arisen from the infilling of faults, 

 fissures, etc., during the denudation by the Trias sea. Eeferring to 

 the hasmatite deposits which occur in the Pennant grit, near Bristol, 

 Mr. Etheridge says,^ "From careful observation and examination, 

 both in this area and others in the district, I have no hesitation in 

 referring the origin of these extensive lode accumulations or deposits 

 of hydrated peroxide, or brown hematite iron ores, to the age of 

 the New Eed Sandstone ; that is to say, the fissures, faults, etc., 

 which occurred in the older rocks (prior to the deposition of the 

 New Red) were filled up, during the period of the accumulation of 

 the Trias group of rocks, either through chemical changes or me- 

 chanical suspension or infiltration." Again, Mr. Etheridge says,^ 

 " I believe, then, that it was during the progress of the denudation of 

 tlie Palgeozoic rocks by the sea of the Triassic epoch that the pre- 

 existing faults and fissures or open joints, etc., along the coast-line 



were mechanically and chemically filled in Doubtless the 



percolation of water through overlying strata highly charged with 

 oxides of iron, as is the case with the New Red Series, has also been 

 a source and mode of accumulation." Though I am disposed to 

 consider it possible that some of the iron was derived from the 



^ Chemical and Geological Essays, p. 13. 



2 Proc. Cotteswold Club, 1865, p. 37. 



2 Quart. Journ. Geol. Society, pp. 185 to 187. 



