C— GEOLOGY. 01 



' The immediate source of iron appears to have been a mantle of 

 Triassic (Keuper) deposits, highly charged with Ferric Oxide, which 

 overlay the denuded edges of the Carboniferous strata. In South Wales 

 the Conglomerates and red marls of the Keuper still cover the ore-field in 

 the Carboniferous Limestone at Llanharry, and carry iron-ore in replace- 

 ment patches in their basal beds. It cannot be doubted that these red 

 rocks originally overspread the whole of the ore-bearing outcrops of the 

 Carboniferous Limestone. In the Forest of Dean Triassic deposits do not 

 touch the Carboniferous basin, but they occur to within a short distance 

 of the southern end at Aylburton, and again in large thickness at no great 

 distance to the east at Newnham on Severn. The former extension of 

 the Trias over the Carboniferous basin may reasonably be invoked to 

 account for the iron-ores by analogy with the condition in South Wales.' 



If this is indeed the explanation of the hsematitic iron-ores of the 

 region, and of the reddening of the limestone, it involves a former extension 

 of the Trias far beyond its present outcrop, and further it implies that the 

 thickness of the deposit was sufficient to allow the formation to climb 

 several hundred feet up the face of the escarpment. There are certain 

 obvious difficulties in the way of accepting this explanation, and it seems 

 to me that it requires further examination. 



The iron-ore deposited around the rock particles of the Trias to which 

 that formation owes its colour is in the dehydrated form of Ferric Oxide, 

 which is extremely insoluble even in acidic waters. In most cases the 

 ore which replaces the limestone is a hydrated Ferric Oxide approximating 

 in composition to Gothite. There is no reason to suppose that the coating 

 of the particles of the Triassic rocks has been dehydrated since it was 

 deposited, so that if the iron-ore has been derived from the Trias this 

 almost insoluble substance must have passed into solution and been 

 reprecipitated. 



Also there are extensive tracts of Carboniferous Limestone, particularly 

 around Chepstow and in the Mendips, which must have been overspread 

 by Trias, and in which no ore is developed, though it is true the rocks may 

 be red-stained ; near Chepstow especially the Trias still rests on the 

 limestone. 



Further, near Llanharry, in the Vale of Glamorgan, the basal beds of 

 the Keuper Conglomerate have locally been converted into an iron-ore 

 similar to that which occurs in the limestone on which the conglomerate 

 rests. It is difficult to suppose that the enrichment was due to iron leached 

 out of the upper part of the conglomerate. From the distribution of the 

 ore bodies it would appear that they occur mainly near the boundary 

 between the limestone and overlying rocks. 



For these reasons it appears to me probable that the ore in the Trias 

 and the ore in the limestone were both derived from the same primary 

 source, that being probably the abundant pyrite which occurs in the Coal 

 Measure rocks of the region in a finely divided form. The destruction of 

 the Coal Measure shales, which in all likelihood stood at that time at a 

 higher elevation than the surrounding limestone, led to the release of the 

 pyrite, which on oxidation gave rise to acidic iron-bearing waters. It was 

 probably due to the activity of such waters in passing over the limestone 

 and accompanying or subsequent oxidation, that the ore in the limestone 



