1870.] HTMERIDGR—BRISTOL DOLOMITIC CONGLOMERATE. 183 
5. InrnvrEnce or Tan 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 Sazdstone, subsequently filled in by the same, and in- 
fluenced during deposition. So also with the rich deposits of hydrated 
peroxides of iron, or brown and red hematitic iron-ores, over and 
around the entire ccal-basin ; for nearly everywhere where the con- 
glomerate rests upon the Carboniferous Limestone, Pennant, or Mill- 
stone Grit do these brown and red heematitic iron-ores exist. ‘The 
area occupied by the breccia due west of Bristol, and on the south- 
west side of the Avon, has always been remarkable, and one of in- 
terest to the mineralogist. The rich hematite-ores of Clapton in 
Gordano, Providence Place, and Ashton, the geodes de. on the flanks 
of Leigh Down, the quartz crystals (Bristol diamonds) and stron- 
tia &e. 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 Carboni- 
ferous 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 Car= 
boniferous Limestone de. 
a, @. Ircn yeins. b. Pocket. 
cement has been remoyed or replaced in many or most instances by 
the infiltration of the oxides of iron, &c., since the deposition of the 
conglomerate. Few inquiries are more interesting or more impor- 
tant than that into the relation which mineral veins hold to the 
rocks which enclose them or with which they are intimately asso- 
ciated*, 
It is well known that the whole of the paleozoic 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 pre- 
viously existing metalliferous veins which traversed the Mountain-limestone. 
Yet, on the other hand, the Zechstein and Alpine limestone are highly metal- 
liferous, and they would appear to be in part the foreign equivalents of the 
dolomitic conglomerates of England. ; 
VOL. XXVI.— PART I. ie) 
