FISSURES IN CLAY IRONSTONES. 
93 
reflux, the ever-advancing change, decomposition, and rc-construction 
which we see everywhere else in Nature, are, althougli slow and silent, 
not least remarkable in the depths of the mineral veins. 
Illustrations of Lodes by the formation of Fissures and the 
Deposition in them of Minerals, in Clay Ironstones. 
Wall-case 24. 
Shelf I. 
1. Clay, or Argillaceous 
Ironstone, of the Coal Measures, 
showing by its smooth surfaces two 
marked directions of cleavage, or 
joint-planes. 
Saundersfoot, near Tcnhij, Pem¬ 
brokeshire. 
2. Clay Ironstone divided by 
similar joint-planes, but making 
oblique angles with each other. 
Merthyr Tydfil, Glamorganshire. 
These argillaceous carbonates of iron 
occur in beds or “ courses” of a few 
inches thick, and partake of the jointed 
character of the rocks above and below 
them. Where, as in the following 
examples, the same substance occurs in 
nodular masses, they are frequently 
veined by cracks arranged more or less 
symmetrically, not visible at the surface 
of the stone, but sometimes occupying a 
large share of the interior. 
3 and 4. Septarian nodule of 
Clay Ironstone, partly sliced 
and polished to show in the interior 
the cracks of contraction after¬ 
wards filled in with Calc-spar, 
cementing occasional angular 
fragments of the stone. 
Shelf II. 
5 and 6. Dark Limestone, po¬ 
lished. 
Fi 'om the Ecton Copper Mine, 
North Staffordshire. 
Presented by the late J. Taylor, F.R.S. 
No. 5 shows in miniature the contor¬ 
tions of the limestone, which occur in 
parts of this mine on a great scale. No. 
6 shows the strata of the same black 
limestone lying horizontally. Both 
masses are intersected by veins of ealc- 
spar, which it will be observed are often 
subject to “ squints,” or small shifts in 
passing from one bed to the other. The 
mine was worked by the Duke of Devon¬ 
shire tothedepth ofnearly 200 fathoms,and 
its great riches in copper were considered 
by local miners to be closely connected 
with the “ saddles” in the limestone. 
7,8, 9, and 10. Pieces of the Iron¬ 
stone called the “ Spotted Vein,” 
at Merthyr Tydjil, exhibiting in 
its cavities, along with pearl-spar 
and a few crystals of quartz, the 
rarer minerals Hatcliettine, and 
Millerite, or Sulphide of Nickel. 
11. Crystals of Galena, on 
Brown Spar, in casts of Anthracosia 
or Cardinia ; illustrating the pro¬ 
duction of metallic sulphides by 
the wet way. 
From the bed of ironstone be¬ 
longing to the Black Shale^’ 
rakefi called the Dim lining,^' 
Chesterfield, Derbyshire. 
12. Longitudinal Fissures of 
Contraction, partially filed with 
Zinc-blende and Brown Spar. 
“ Old Man’’ measure, Black Shale 
Ironstone, Chesterfield. 
13. Section of the Stem of a 
Tree, of which the outer rind is 
converted into Bituminous Coal, 
showing open cracks of contrac¬ 
tion in the interior. 
Clay Balls Ironstone, Dudley, 
Wore ester sh ire. 
14. Fissures of Contraction, 
some of which are left unfilled, 
generally widest in the middle of 
the thickness of the nodule. 
“ Old Man” Ironstone, Black 
Shale Bake, Chesterfield. 
For particulars of the characters and position of this very curious and produc¬ 
tive group of ironstones, see the “ Iron Ores of Great Britain,” Part I, published 
among the Memoirs of the Geological Survey. 
