5. Influence of the Conglomerate upon the production of Minerals.
The well-known rich deposits of calamine (carbonate of zinc) once extensively worked at Shipham, Burrington, Rowborrow, Chewton, Priddy, and Heydon, on the Mendip Hills, occurred chiefly in the mass of the dolomitic conglomerate itself, but also in faults, fissures, and hollows, or pockets, in the Carboniferous Limestone, which were present prior to the deposition of the conglomerate and New Red Sandstone, subsequently filled in by the same, and influenced during deposition, oo also with the rich deposits of hydrated peroxides of iron, or brown and red haematitic iron-ores, over and around the entire coal-basin ; for nearly everywhere where the conglomerate rests upon the Carboniferous Limestone, Pennant, or Millstone Grit do these brown and red haematitic iron-ores exist. The area occupied by the breccia due west of Bristol, and on the southwest side of the Avon, has always been remarkable, and one of interest to the mineralogist. The rich haematite-ores of Clapton in Gordano, Providence Place, and Ashton, the geodes &c. on the flanks of Leigh Down, the quartz crystals (Bristol diamonds) and strontia &c. are everywhere associated with the conglomerate when it rests on the older rocks before mentioned.
Nearly all, if not all, the iron-ore in veins or faults in the Carboniferous Limestone, Millstone Grit, and Pennant were filled in at the time when the conglomerates were being developed and deposited, and these older rocks denuded away by the Keuper sea. So with the ores in pockets, where the carbonate of lime and magnesian
Fig. 3. — Section showing mode of occurrence of Iron-ores in Carboniferous Limestone &c.
a, a. Iron veins.
b. Pocket.
cement has been removed or replaced in many or most instances by the infiltration of the oxides of iron, &c, since the deposition of the conglomerate. Pew inquiries are more interesting or more important than that into the relation which mineral veins hold to the rocks which enclose them or with which they are intimately associated*.
It is well known that the whole of the palaeozoic rocks in this area, as in others, were placed in their disturbed position prior to
- The ores of zinc and lead may have been derived mechanically from previously existing metalliferous veins which traversed the Mountain -limestone.
Yet, on the other hand, the Zechstein and Alpine limestone are highly metalliferous, and they would appear to be in part the foreign equivalents of the dolomitic conglomerates of England.
VOL. XXVI. PART I. O
