currents bearing dark mud once more invaded this part of the sea and threw down the material that now forms the band of shale (4). The absence of organic remains in this band probably shows the inroad of mud to have destroyed the life that had previously been prolific. When this condition of things had been brought about, renewed volcanic explosions took place in the neighbourhood. First came showers of dust, ashes, and stones, which fell over the sea, and are now represented by the band of tuff (5). Then followed the outpouring of a stream of lava (6), with its characteristic cellular structure. But this did not quite exhaust the vigour of the volcano, for the band of tuff (7) points to successive showers of dust and stones. When the explosion ceased, the deposition of dark mud, which had been interrupted by the volcanic episode, was resumed, and the band of shale (8) was laid down. From the fragments of ferns and other plants in this shale it is clear that land was not far off. The sea had evidently been gradually shallowing by the infilling of sediment and volcanic materials, and at last, on the muddy flat, represented by the layer of fire-clay (9), marshy vegetation sprang up into a thick jungle like the mangrove-swamps of tropical shores at the present day. But after growing long enough to form the bed of matted vegetable matter now represented by the coal-seam (10), the verdant jungle was invaded by the sea, and sank under the muddy water that threw down upon its submerged surface the grey shale (11). In this shale we detect interesting traces of the renewal of volcanic activity, more especially in occasional large blocks of lava, which have evidently been ejected by some volcanic explosion, as in the example already cited on p. 102 (Fig. 38). A more vigorous volcanic outburst poured out the stream of columnar lava (12) which buried the whole and forms the top of the section.

Veins and Dykes. These have already been referred to in Chapter IX as part of the evidence for volcanic action. We have here to consider how they occur in connection with the protrusion of eruptive material within the crust of the earth. Where the material so erupted has solidified in a vertical or nearly vertical fissure so as to form a wall-like mass, it is called a dyke (Fig. 43 and d in Fig. 107). Otherwise the portions of erupted rock that have consolidated in irregular rents are known as veins.

Veins are of common occurrence round bosses of granite, where they can be traced into the parent mass from which they have proceeded (Fig. 106). They may likewise be observed in con

nection with intrusive sheets and bosses of basalt and diorite, from which they ramify outwards into the surrounding rocks. Their occurrence there is one of the proofs of the intrusive character and subsequent date of such sheets (p. 204).

Dykes vary from less than a foot to 70 feet or upward in breadth, and run in nearly straight courses sometimes for many miles. They consist most usually of diabase, andesite, basalt, or allied rock. Sometimes they have risen along lines of fault; but in hundreds of instances in Great Britain, they do not appear to be connected with any faults, but actually cross some of the largest

FIG. 110. Map of Dykes near Muirkirk, Ayrshire. 1. Silurian rocks. 2. Lower Old Red Sandstone. 3. Carboniferous rocks. f, f,f. Faults. d, d. Dykes.

faults in the country without being deflected. The remarkable way in which dykes have risen through a complicated series of rocks and faults and have preserved their courses is exemplified in Fig. 110.

Like intrusive sheets, but in a less degree, dykes harden or otherwise alter the rocks on either side of them; they likewise present a similar closeness of grain along their margins where the molten rock was most rapidly chilled by coming in contact with the cold walls of the fissure. Sometimes, indeed, their sides are coated with a thin crust of black glass, as if they had been painted with tar. This glass represents the effect of rapid cooling (see Basalt-glass, p. 165). No doubt the whole rock of the dyke, at the time when it rose from below and filled up the space between the two walls of its opened fissure, was a molten glass. The portions that were at once chilled by contact with the walls adhered as a layer of glass. But inside this layer, the molten rock had more time to cool. In cooling, its various minerals crystallised and the present crystalline structure was developed. But even yet, though most of the rock is formed of crystalline minerals, portions of the original glass may not infrequently be detected between them when thin sections are placed under the microscope (p. 144).

Necks. These are the filled-up pipes or funnels of former volcanic vents. Their connection with volcanic action has been already alluded to on p. 106. They are circular or elliptical in ground-plan, and vary in diameter from a few yards up to a mile or more (see Figs. 40, 41, 42). They consist of some form of lava (quartz-porphyry, basalt, diorite, etc.) or of the fragmentary materials that, after being ejected from the volcanic chimney, fell back into it and consolidated there. They occur more particu

6

a

n

a

EC

FIG. 111.-Section of a volcanic neck. The dotted lines suggest the original form

of the volcano.

larly in districts where beds of lava and tuff are interstratified with other rocks. The necks represent the vents from which these volcanic materials were ejected. In Fig. 111, for example, the beds of lava and tuff (bb) interstratified between the strata a a and c c have been folded into an anticline. In the centre of the arch rises the neck (n) which has probably been the chimney that supplied these volcanic sheets, and which has been filled up with coarse tuff, and traversed with dykes and veins of basalt (*). The dotted lines, suggestive of the outline of the original volcano, may serve to indicate the connection between the neck and its volcanic sheets, and also the effects of denudation.

Necks are frequently traversed by dykes (* in Fig. 111), as we know also to be the case with the craters of modern volcanoes. The rocks surrounding a neck are sometimes bent down round it,

as

if they had been dragged down by the subsidence of the material filling up the vent; they are also frequently much hardened and baked. When we reflect upon the great heat of molten lava and of the escaping gases and vapours, we may well expect the walls of a volcanic vent to bear witness to the effects of this heat. Sandstones, for instance, have been indurated into quartzite, and shales have been baked into a hardened clay or porcelain-like substance.

Mineral Veins.

Into the fissures opened in the earth's crust there have been introduced various simple minerals and ores which, solidifying there, have taken the form of Mineral Veins. These materials are to be distinguished from the eruptive veins and dykes above

described. A true mineral vein consists of one or more minerals filling up a fissure which may be vertical, but is usually more or less inclined, and may vary in width from less than an inch up to 150 feet or more. The commonest minerals (or veinstones) found in these veins are quartz, calcite, barytes, and fluor-spar. The metalliferous portions (or ores) are sometimes native metals (gold and copper, for example), but are more usually metallic oxides, silicates, carbonates, sulphides, chlorides, or other combinations. These materials are commonly arranged in parallel layers, and it may often be noticed that they have been deposited in duplicate on each side of a vein. In Fig. 112, for instance, we see that each wall (w w) is coated with a band of quartz (1, 1), followed successively by one of blende (sulphide of zinc, 2, 2), galena (sulphide of lead, 3, 3), barytes (4, 4) and quartz (5,5). The central portion of the vein (6) is sometimes empty or may be filled up

P

with some veinstone or ore. Remarkable variations in breadth characterise most mineral veins. Sometimes the two walls come together and thereafter retire from each other far enough to allow a thick mass of mineral matter to have been deposited between them. Great differences may also be observed in the breadth of the several bands composing a vein. One of these bands may swell out so as to occupy the whole breadth of the vein, and then rapidly dwindle down. The ores are more especially liable to such variations. A solid mass of ore may be found many feet in breadth and of great value; but when followed along the course of the vein, may die away into mere strings or threads through the veinstones.

The duplication of the layers in mineral veins shows that the deposition proceeded from the walls inwards to the centre. In the diagram (Fig. 112) it is evident that the walls were first coated with quartz. The next substance introduced into the vein was sulphide of zinc, a layer of which was deposited on the quartz. Then came sulphide of lead, and lastly, quartz again. The way in which the quartz-crystals project from the two sides shows that the space between them was free, and, as above stated, it has sometimes remained unfilled up.

There appears to be now no reason to doubt that the substances deposited in mineral veins were mainly introduced dissolved in water. Not improbably heated waters rose in the fissures, and as they cooled in their ascent, they coated the walls with the minerals which they held in solution. These minerals may have been abstracted from the surrounding rocks by the permeating water; or they may have been carried up from some deeper source within the crust. During the process of infilling, or after it was completed, a fissure has sometimes reopened, and a new deposition of veinstones or ores has taken place. Now and then, too, land-shells and pebbles are found far down in mineral veins, showing that during the time when the layers of mineral matter were being deposited, the fissures sometimes communicated with the surface.

Summary. In this chapter it has been shown that, in many cases, the rents in the earth's crust have been filled up with mineral matter introduced into them, either (i) in the molten state, or (ii) in solution in water. (i) The forms assumed by the masses of eruptive rock injected into the crust of the earth have depended upon the shape of the fissures into which the melted matter has been poured, as the form of a cast-iron bar