Lavernock Point, will illustrate minor complications of fracture, and a bending of certain of the beds acted upon, m, m, m, being minor parts of the same dislocation which has traversed earthy dolomitic limestone and marl a; varieties of dolomitic limestone, b, c, d, e, and f; dolomitic conglomerate, g (all these of the new red sandstone series); and lias 7. The beds at h correspond with those on the left. While the fractures have merely broken the former deposits, the edges of the lias have been turned up, as if by a certain amount of lateral pressure. In some faults this turning up of a portion of the beds acted upon, occasions the observer to suspect that, after the fracture, there has been some settlement from an upraised position (for the time), producing the needful friction, even for upturning the edges of beds on the under part of a fault, as shown at m on the right of the section (fig. 283). Usually the side relatively lowered is found raised at the edge in an inclined fault, the consequent friction turning up the end of the superior rock conformably with the movement.

As a vertical section may only give the apparent movement of the parts of rocks fractured and faulted, it is desirable that the observer should search for the direction of any friction-marks attending pressure of the rocks on one side against those on the other, in order to discover that in which the movement has really been effected. This investigation will sometimes lead him to find that, though the general plane of a fault may dip in a given direction, the movement has not always corresponded with it. Some of these friction-marks bear evidence of the action of enormous pressure, more especially in those cases where the dislocation may, in its plane, amount to several thousand feet, and yet the rocks thus moved against each other, and once so far asunder, be now closely jammed together. The contents of dislocations, whether known as common faults or mineral veins, often present beautiful impressions of these friction-marks, parts of the walls of the fractures, after grating against each other in their movement, having finally left cavities in which various mineral substances were accumulated, taking the form of the surfaces against which their first deposit was effected.

CHAPTER XXXV.

FILLING OF FISSURES AND OTHER CAVITIES WITH MINERAL MATTER.— SULPHURETS OF LEAD, COPPER, ETC., REPLACING SHELLS.-FILLING of MINOR FISSURES.-SOLUBILITY AND DEPOSIT OF MINERAL MATTER IN FISSURES. SOLUBILITY OF SULPHATE OF BARYTA. DEPOSITS FROM SOLUTIONS IN FISSURES.-EFFECTS PRODUCED IN HEATED FISSURES BENEATH SEAS.-MANY SIMILAR SUBSTANCES FOUND IN MINERAL SPRINGS AND VEINS.-FREQUENT OCCURRENCE OF SULPHUR, ARSENIC, ETC., WITH CERTAIN METALS IN MINERAL VEINS.-ACTION AND REACTION OF SUBSTANCES UPON EACH OTHER IN FISSURES AND CAVITIES.-CHARACTER OF METALLIFEROUS VEINS AMID ASSOCIATED DISSIMILAR ROCKS.-CONDITION OF MINERAL VEINS TRAVERSING ELVAN DYKES IN CORNWALL. INFLUENCE OF THE DIFFERENT ROCKS TRAVERSED ON THE MINERAL CONTENTS OF FISSURES.-MODE OF OCCURRENCE OF LEAD ORES AMID THE LIMESTONES AND IGNEOUS ROCKS OF DERBYSHIRE.-'FLATS' OF LEAD ORE IN LIMESTONE DISTRICTS.—METALLIFEROUS DEPOSITS IN THE JOINTS OF ROCKS.-RELATIVE DIFFERENT DATES OF MINERAL VEINS.

THE filling of fissures and other cavities with mineral matter may, to a certain extent, be considered as in part connected with the changes and modifications of rocks above mentioned; since from the filling of minor cavities and fissures, such as occur in or traverse small portions of an accumulation, whether of igneous or aqueous origin, much change or modification may arise in the containing rocks. The filling of cavities, such as those previously noticed in vesicular lava and molten matter of all geological times, converting a highly porous and often originally light substance into a very solid rock, effects a marked change of structure. The infiltrations of the mineral substances into the cavities, in these cases, become important in the consideration of those which have filled various fissures and dislocations, as well as cavities of far greater size, since they seem to point to the solution of some substances, or of the elementary matter composing them, and to the power of such solutions to traverse the pores of rocks, even of those which

are considered very solid and compact, in a manner which, at first sight, might not be expected. Let the observer, for example, study certain of the nodules of the impure carbonate of iron, known as clay ironstones, in many of the localities where they are obtained from the coal measures of the British Islands, opportunities for which are abundant in South Wales, Monmouthshire, Staffordshire, Derbyshire, and elsewhere. While in many of these nodules, the cracks, when they present themselves, as they often do, in the manner mentioned previously (fig. 227, p. 597), only contain more pure carbonate of iron or are entirely empty, at others they are incrusted or filled with such substances as copper pyrites, and the sulphurets of lead, zinc, nickel, and iron, with the occasional occurrence of other minerals of a different class. In such cases the observer can have little doubt that the component parts of these substances have come, by infiltration from without, into the cracks of the nodules of impure carbonate of iron, through their exterior pores, and through those and the lamina of the surrounding argillaceous shales. He is therefore prepared to infer that these bodies, or their component parts were in a soluble state when they entered the cavities formed by the cracks in the nodules.

When he examines the minerals which have, under certain conditions, replaced organic remains in various rocks, the geologist may still further be prepared to regard the matter of these and other compound substances as having been introduced in solution into cavities left by the decomposition and disappearance of mollusc shells, or other organic bodies. Copper pyrites has been found to replace the shells of spirifera, at Doddington, Somersetshire*sulphuret of lead various cavities left by the shells of molluscs in the lias near Merthyr Mawr, Glamorganshiret—and sulphate of baryta portions of corals in the mountain limestone of Cromford, Derbyshire. Sulphuret of iron very frequently occupies the places of mollusc shells in many rocks, especially those which are argillaceous, even insinuating itself amid the matter of fossil bones, such as those of saurians in the lias, and other deposits. Silica, as

In this locality there was a vein of copper. The ores raised were principally green and blue carbonates, and were first obtained in the new red sandstone conglomerate of the locality above a vein in the Devonian rocks beneath. Horner, Trans. Geol. Soc. London, vol. iii., pp. 352 and 363.

The sulphuret of lead is much disseminated in this part of South Wales, and often in cavities. It occurs in the cracks of fossil wood in the lias near Dunraven Castle, in the same manner that the sulphuret of iron is often seen in coal beds, and in fossil wood in numerous clays of different geological dates.

This fact is interesting in connexion with the considerable quantity of sulphate of baryta found in the lead veins and other cavities of that part of Derbyshire.

might be expected also, occupies the cavities left by shells, of which the chalcedonic replacements of the various shells of the greensand series at Blackdown, Devon and Somerset, are beautiful examples. Even the carbonate of lime of many fossil mollusc shells does not always appear to be that of the original, but to have been infiltrated into cavities left upon the disappearance of the matter of the actual shell, the particles of the carbonate of lime not being adjusted in the manner they usually are in shells of the same class by living animals, but as they would be upon simple infiltration and crystallization in any cavity. Again, in the crystals of felspars decomposed in the body of a rock, the original substance of the crystals removed, and replaced by peroxide of tin, even part of the original felspar crystal sometimes remaining, while the rest of its form is replaced by the peroxide of tin, as in an elvan at St. Agnes, Cornwall, the observer has another example of the inflow of mineral matter in solution into cavities and through the pores of the rock in which such cavities may be situated. In fact, looking at the subject generally, the various cavities in the rocks composing the crust of the earth, have a tendency to be filled by mineral matter, the component parts of which find their way to them in solution.

Passing from these cavities to those produced by cracks, these of minor size, and confined either to one, two, or some small number, of beds of sedimentary deposits, or some very limited volume of an igneous accumulation, it would be expected that, as a whole, the matter infiltrated into such cracks would chiefly partake of the mineral character of the rocks so broken, so that the substances principally filling the cracks in limestones would be calcareous, while those amid siliceous rocks would be quartzose, as is usually the fact. From the prevalence, however, of particular conditions, quartz veins are occasionally found in limestones, and calcareous matter among the siliceous rocks. This usually occurs when the limestone beds form a very subordinate portion of a sandstone or argillaceous accumulation, chiefly composed of silicates, or when calcareous deposits predominate among those of other kinds; as, for example, is the case with the igneous rocks of Derbyshire, where the vesicles and minor veins of the latter are often filled with calcareous spar.*

* The filling of cavities and small fissures in igneous rocks by carbonate of lime is not unfrequent, even when cal areous rocks do not constitute any very large proportion of a general mass of mixed accumulations. Thus at Trecarrell Bridge, between Launceston and Tavistock, the highly-vesicular rock of that locality, contempora

Proceeding to examine the filling of cavities and fissures of larger dimensions, and such as, not confined to minor volumes of rocks, can be traced for considerable distances, and the depths of which are unknown, an observer will have not only to bear in mind the incrustations of the sides of such fissures by the substances, which, passing amid the pores or small fissures of rocks on the minor scale, are ready to fill up or incrust any cavities presenting themselves, no matter of what kind or how formed, but also to consider the kind of substances, and their mode of action upon each other, which may be derived from various distances and sources. Viewing a considerable fissure, in its simple form, somewhat vertically traversing various beds of dissimilar rocks, as in the following section (fig. 284), a to f, each affording some different Fig. 284.

e

or variously combined matter in solution, and confining his attention, at first, to solutions, the geologist has a more complicated problem presented to his attention than the mere infiltration of mineral matter through the pores of rocks into small cavities and fissures in them. He has to regard not only the probable combinations and decompositions effected by a mixture of substances introduced into the fissure, but also the motion of the whole of the liquid in it, according to temperature. The fissure may either be one through which waters rise to the surface of land, and overflow it, thus discharging large volumes of water containing mineral matter in solution of various amount and kind, or the liquid may merely rise to such a height in the fissure as to remain confined to it, and the portions of rocks adjacent, amid the pores and interstices of which it may also enter. According to temperature also will he

neously formed with the Devonian rocks amid which it occurs, is rendered solid by the infiltration of carbonate of lime from adjacent calcareous beds of no great purity or importance. The ready solubility of the carbonate of lime, when sufficient free carbonic acid is present, has occasioned the passage of the former substance from the calcareous beds into the vesicles of the igneous and juxtaposed rock, and its deposit there, when unless decomposed and again removed, it would prevent the deposit of other substances passing in solution through the pores of the rock.