252 - THE GEOLOGIST. 



other minerals have their distinctive " gozzans " and surface associa- 

 tions of particular minerals. This " clod " may either be a ferruginous 

 clay or a ferruginous marl, according as the gangue of the ore is sili- 

 ceous or calcareous. In the Llantrissant district, the matter being 

 chiefly silica, we have the " clod " in the form of a yellow clay (the 

 " native ochre " of the painter), but nevertheless containing lime : in 

 the Forest of Dean the ores are calcareous, and the " clod " is a true 

 marl, highly charged with small fragments of compact hsematite, or 

 " black brush." 



The exact geographical range of the iron stone- deposits contiguous 

 to the boundary of the coal-measures, has not yet been ascer- 

 tained — although there has as yet been no lack of enterprise to 

 make discoveries. In the Llanharry district, which immediately 

 adjoins Llantrissant, a score and more of trial pits testify the 

 eagerness of the search ; and wherever this has been rewarded, the 

 ore has been found similarly situated with respect to geological 

 position, that is, near the axis of some anticlinal of the limestone. 



The common mining-axiom, that the depth from surface and wealth 

 of a vein are conterminous facts, is certainly not founded on any 

 precedent derived from the occurrence of those deposits of hsematite- 

 ore which lie (as it were) bedded. Indeed, unlike copper or tin, or 

 even lead, of which, as a vein or lode " holds down " in depth, it 

 assumes a richer mineralisation and more constant ore-bearing charac- 

 ter, iron-ore in veins is not generally found in deposits of the same 

 magnitude in depth as near the surface j and more especially when 

 the run of the veins is horizontal. In the Forest of Dean, where this 

 horizontal or "bedded" character of the deposits is almost typical, it 

 is extremely doubtful that, if the centre of the basin were proved by 

 deep " winnings," the ore would be found in the same quantity as 

 near the basset of the " measures ; " and, taking the same view, 

 I believe that any sinkings through the dolomite to the synclinal 

 troughs of the limestone at Llantrissant and Llanharry would be, both 

 geologically and economically, failures. But it must be understood 

 tliat by deep " winnings" I do not mean the actual depth at which a 

 shaft may intersect a vein of the ore, since, although actually very near 

 to the crop, certain deposits may be carried deep from surface by the 

 high angle at which the strata may be raised. My restriction of the 



