304 
TIIU GEOLOGIST. 
ill piirt, couvertcd into silica. Most of these siliceous beds, however, 
are composed of a description of gTanular calcedony, or cliei-t, com- 
monly of a dark grey colour, inclining to and passing into black, 
semi-opa(jue, and having its surface and fissures often lined with 
minute pyramidal crystals of quartz. These chert beds do not 
appear to have any du-eet connection with the ore-deposits, but theii- 
history, in other respects, as I shall presently show, may undoubtedly 
have a bearing in the general question of the origin of the various 
phenomena embraced by the dynamical theories involved in om* 
subject. 
But we must now turn to the metallic contents of the fissui'es. 
The pipe- as well as the rake-veins, are invariably mineralized with 
sulphuret of lead (galena oi'e), mixed with a small quantity of sul- 
phui'et of zinc (blende), which is deposited in the ordinaiy layer-like 
form on the walls of the fissure, and accomjjanied by a gangue or 
rain-stone of carbonate of lime, barj^tes-calcite (cai'bonate of lime and 
baryta), cawk, or the massive variety of sulj^hate of baryta, dreelite 
(sulphate of barj'ta and hme) and strontianite. Sometimes, never- 
theless, the pipe-veins enclose large separate frag-ments or lumps of 
ore embedded in soft decomposed limestone, and unaccompanied by 
any true veinstone. Such occuiTences are not infrequent where the 
veins squat — that is a line, after passing from one set of beds to ano- 
ther set situate below them, the pipe swells out in a direction parallel 
Avith the com'sing of the last entered stratum. (See the section of 
the pipe-vein in the diagTam fig. 3.) When the pipes come do^vn to 
the saddle-beds, they commonly expand in size rapidly, the quantity 
of ore increases, and the vein assumes a more banded structure, the 
vein-stone alternating with ribs of ore ; at the same time their is an 
augmented flow of water, and the adjoining I'ock, or country, is 
thickly tlu'eaded with small ore-bearing fissures. At such junctions 
lai'ge deposits are the rule — that is to say, if the pipe has previously 
borne ore ; and any veiy considerable returns are seldom expected 
until these points are reached, unless, as is I'arely the case, the rake- 
veins are found to be veiy rich at their intersections of the jiipe-veins 
above the saddles. The first appearance of the contortions is marked 
by a universal fissui'ing of the rock, most of the fissures bearing ore, 
and the ceasing ot the pipe to retain any longer the distinctive cha- 
racter of a sepai'ate vein. 
A rude representation of the arrangement of the ore in the saddles 
after their intersection by the vein is given in fig. 4. A marks the 
btickle or crown of the saddle, which is usually unmetalliferous ; c c' 
are the \rings, and on them are some of the principal deposits of ore, 
a a' ; B is the saddle-joint, and is commonly ore-bearing for a limited 
distance, vertically between the huckle and the trough, the ore djnng 
out below ; the troughs are marked//', and are the parts richest in 
ore, a a', very large and solid deposits being often encountered at 
these points ; f !>' is the trough-joint, usually more productive than 
tlic saddle-joint ; lastly, v\ x, ?/, z are mineralized fissures i-unning up 
to llic iiii)e, and often containing ore solid enough to be worth follow- 
