364 



THE GEOLOGIST. 



in part, converted into silica. Most of these siliceous beds, however, 

 are composed of a description of granular calcedony, or chert, com- 

 monly of a dark grey colour, inclining to and passing into black, 

 semi-opaqne, and having its snrface and fissures often lined with 

 minnte pyramidal crystals of qnartz. These chert beds do not 

 appear to have any direct connection with the ore-deposits, but their 

 history, in other respects, as I shall presently show, may nndoubtedly 

 have a bearing in the general question of the origin of the various 

 phenomena embraced by the dynamical theories involved in our 

 subject. 



But we must now turn to the metallic contents of the fissures. 

 The pipe- as well as the rake-veins, are invariably mineralized with 

 sulphuret of lead (galena ore), mixed with a small quantity of sul- 

 phm*et of zinc (blende) , which is deposited in the ordinary layer-Hke 

 form on the walls of the fissure, and accompanied by a gangue or 

 rain-stone of carbonate of lime, barytes-calcite (ca;rbonate of lime and 

 baryta), cawk, or the massive variety of sulphate of baryta, dreelite 

 (sulphate of baryta and lime) and strontianite. Sometimes, never- 

 theless, the pipe-veins enclose large separate fragments or lumps of 

 ore embedded in soft decomposed hmestone, and unaccompanied by 

 any true veinstone. Such occurrences are not infrequent Avhere 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 

 with the coursing of the last entered stratum. (See the section of 

 the pipe-vein in the diagram fig. 3.) When the pipes come down 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 rock, or country, is 

 thickly threaded with small ore-bearing fissures. At such junctions 

 large deposits are the rule — that is to say, if the pipe has previously 

 borne ore ; and any very considerable returns are seldom expected 

 until these points are reached, unless, as is rarely the case, the rake- 

 veins are found to be very rich at their intersections of the pipe-veins 

 above the saddles. The first appearance of the contortions is marked 

 by a universal fissuring of the rock, most of the fissures bearing ore, 

 and the ceasing of the pipe to retain any longer the distinctive cha- 

 racter of a separate 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 

 buckle or crown of the saddle, which is usually unmetalhferous ; c c' 

 are the wings, 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 dying 

 out below ; ihe troughs are marked//', and are the parts richest in 

 ore, a a', very large and solid deposits being often encountered at 

 tliese points ; D I)' is the trough-joint, usually more productive than 

 the saddle-joint ; lastly, v:, ,t, y, z are mineralized fissures running up 

 to the pipe, and of\en containing ore solid enough to be worth follow- 



