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have been traced through different beds in the same instance, and in different localities have been observed traversing some of all the limestone strata above the upper sandstone. I have observed such sheets followed to the depth of 80-90 feet through different beds of the upper magnesian, and at the lowest depth still continued, sometimes increasing in thickness. Others are reported to have been followed to the depth of considerably more than 100 feet and left still going down.*
When the crevice is of much width, and its walls are nearly parallel, it is called a crevice opening. The space, traversed by two or more connected sheets, might be called such; but the term is usually applied to an opening of a foot or more in width, in which the mineral occurs in some other form than that of a sheet. Such openings are nearly always quite vertical, but occasionally local pitches occur. The walls of such openings are rarely strictly parallel, but there is usually a series of enlargements and contractions. This tendency to enlargement and contraction is common, and is accompanied more or less by lateral cavities of differ- · ent size and form. Indeed it may be said that those openings, which continue with little variation in width to a great extent, vertically or in the direction of their course, are one extreme, and that a series of isolated openings or cavities (called pockets,) connected by mineral seams, such as have been mentioned, are the other, between which almost every degree of alternate enlargement and contraction may be found. Openings are more rarely found of much extent vertically than in the direction of their course. Thus in sinking on a crevice, different openings will be found, one beneath another, little interrupted in the direction of their course, but generally separated from each other by close rock, traversed only by a mineral seam, yet occasionally connected in part by long narrow crevices, or by shorter and wider passages; the last sometimes rising to a greater or less height above the
* The largest North and South sheet at the East Blackleg Diggings is said to have been followed down to the depth of 140 feet, at the engine shaft, and left still going down, although with diminished thickness.
upper opening, and then called chimneys. In some instances, instead of this series of openings, one beneath the other, separated, by close rock, there is only a series of enlargements, corresponding to the openings, separated by alternate contractions; the crevice remaining open throughout the descent. Different ranges in the same group occasionally differ in this respect; one being marked by distinct openings, and another adjacent, only by enlargements and contractions. Different ranges are also distinguished in the same manner, in the direction of their course; the openings in one presenting a series of isolated cavities or pockets, in that difection, separated by close rock, marked by a mineral seam, and in another, only alternate enlargements and contractions. Whenever, in such cases, the pockets or enlargements rise to a considerable height above the range of the opening, they are also called chimneys.
These are the most usual forms assumed by the vertical open. ings in the upper part of the upper magnesian. They commence at different depths in the rock, sometimes near or at its upper surface, sometimes at the depth of many feet. Where the whole thickness of the upper magnesian is present, together with the overlying blue shale or pipe clay, I have never seen the crevices or openings penetrate the latter, or even the thin bed of schistose limestone, called shingle rock, sometimes overlying the thicker layers of the upper magnesian. But often the crevice is struck immediately on entering the thicker layers of that rock, and the opening soon after, and in some instances, I have observed the openings rise to its upper surface, and immediately overlaid by the pipe clay or blue shale. Where these or the upper part of the upper magnesian have been denuded, such openings reach to the surface of the rock, and are called open crevices. More generally, although the crevice may at times be struck at little depth in the rock, the opening is not reached till at a greater depth, which in each group is usually common to all the ranges. This may be called the level of the openings, and it is at this depth,
known by experience in the different localities, that openings are expected.*
The openings sometimes gradually expand from a parrow crevice, but more usually terminate above in a low arch, or are flatroofed. The rock immediately above the opening is called the cap, and when one opening lies below another, the rock separating them is the cap of the lower. It has been already stated, , that the rock immediately adjoining the openings is harder than the rock generally. This is particularly true of the cap, and when in sioking on a crevice, the rock becomes unusually hard, an opening is expected.
The openings, now under consideration, are usually filled with soft and loose materials, which seem to have been formed by the decomposition of the rock originally occupying them. These are usually what are called sand, clay and tumbling rock; the sand derived from the decomposition of the limestone; the clay, from that of shale or claystone; while the tumbling rock is but the harder and more compact portion of the limestone, which has resisted decomposition. In examining these materials, I have almost invariably found the sand and tumbling rock conforming distinctly, in their arrangement, to the stratification of the limestone, and the clay either arranged as distinctly in the same order, or appearing as an original matrix of the mineral.
I have already stated that the term opening is also applied to
* The crevices are not only interrupted abure by the blue shalo and shingle rock, but often by many foet of the upper magnesian, and are sometimes struck only at a short distance abore the opening. A mineral crevice usually first shows an iron staia on its walls, and lower down a sean of clay or hematite (iron rust,) and often still ncarer the opening, a sheet of mineral, or detached pieces of the same in a sheet or vein position, leading to the opening. Often a scam of black ochre (oxyd of manganese) preced(s the mineral, indicating its near approach, and the latter, when first met, is usually more or less coated with the carbonate. Not only is it common to find a seam of clay bordering sheets and veins, or otherwise investing the mineral as a matrix, but I hare observed flat-roofed or low-arched vertical openings lined by a smooth unbroken seam of joint clay, more or less completely investing them, and yet the materials inclosed, except the mineral and its immediate matrix, arranged conformably to the stratification, and apparently altered or modified portions of the rock.
limited portions of the rock, less disintegrated, marked by certain peculiar characters, and traversed by the mineral, or through which it is disseminated. In such instances, other substances, besides the mineral, may traverse the rock, or be disseminated throngh it, such as other metallic ores, clay, calcareous spar and sulphate of barytes. Iron pyrites is always originally present in such portions of rock, and has generally suffered more or less decomposition, leading to the disintegration of the rock, and to the ferruginous stain common to all openings. The limestone, in such openings, even when least altered, appears to be made up of hard compact concretions, little or not at all subject to stain or disintegrate, imbedded in a gronnd of more granular structure, more or less subject to stain and disintegrato from disseminated pyrites. When this part of the rock is stained, as is usual, the rock of the opening has a peculiar mottled appearance, and is called calico rock, in some localities. This is peculiarly characteristic of the flat openings in the lower beds of the upper magnesian, particularly in the fint bed. In the vertical openings in the upper part of the upper magnesian, the tumbling rock corresponds to the harder unstained nodules or concretions in the calico rock, but usually of a much larger size, and the sand to the stained and softened ground of the lattor.
In the vertical openings in the upper part of the upper magnesian, the mineral, in general, is arranged vertically. In these openings, it shows a greater or less tendency to assume its regular cubic form. When its form is more regular, it is called square mineral; and when a number of cubes are combined, particularly in a sheet, it is called cog mineral. When its form is more irregular, showing only an approach to its regular cubic form, but in more or less detached masses, it is called chunk mineral.
The cubes or more irregular forms are arranged, in the vertical openings, in a certain order, more or less distinct, which may
be called the Vein order. This is most distinct in the East and West ranges, but may be traced more or less even in the North and South sheets, where an approach to the cubic form is observable, and
may be also recognized in the arrangement of the mineral in the flat openings. In this order, the cubes or masses deviate from a a direct line,alternately to the right and left, forming a zig-zag, but in such a manner as to continue the general direction. When a crevice is of little width, it is usually traversed by a single vein, or course of mineral in vein order, usually accompanied by clay as its matrix. But if this be examined strictly, it will be generally found double, or divided by a middle seam into two series of cubes or less regular forms, and the same is equally true of the sheets, which, as I have observed, occasionally in the wider parts of their crevices approach the regular form of the mineral. This too is often observed where the sheets are met by cross crevices. When a narrow crevice widens, the single vein divides, each of its symmetrical parts being continued along its wall, or sometimes only one of them, the other being interrupted. The surface of the mineral next the wall is then less regular, and conforms in general to the surface to which it adheres; that towards the middle of the crevice, which is usually occupied by clay, is more reg. ular; the whole vein, in this instance, forming a more or less perfect geode. Where the crevice alternately widens and contracts, the same alternation will be observed in the arrangement of the vein. Such geodes or more irregular deposits, in the enlarged portions of the vein, are called bunches. In some veins there is a greater tendency to form bunches than in others, and in such cases the intervening
portion of the vein is usually diminished or even interrupted. The arrangement of the vein thus corresponds to that of the openings.
Where the opening is wide, and includes considerable masses of tumbling rock, it may contain several such veins or courses of mineral, separated by the masses of rock, which may either unite, or be connected by smaller cross veins. Sometimes the wider vertical openings are traversed longitudinally, to a greater or less extent, by one or more vertical masses of rock, called key-rocks; but these rarely divide the openings completely, but are more or less insulated, corresponding to the horses of English