margins where the molten rock was most rapidly chilled by coming in contact with the cold walls of the fissure. Some

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.

times, 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. 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.

NECKS.-These are the filled-up pipes or funnels of former volcanic vents. Their connection with volcanic action has been already alluded to in chapter ix. They

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are circular or elliptical in ground-plan, and vary in diameter from a few yards up to a mile or more. 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 particularly 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

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

(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 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 quartzite, calcite, barytes, and fluor-spar. The metalliferous portions (or ores) are sometimes native metals (gold and copper, for example), but more usually are 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 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 have allowed a thick mass of mineral matter to

be 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 quartzcrystals 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 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 is regulated by that of the mould into which the melted metal is allowed to run. Taking this principle of arrangement, we find that eruptive rocks may be grouped into (1) Bosses, or irregularly-shaped masses, which have risen through irregular fissures, and now, owing to the removal of the rock under which they solidified, form hills or ridges. The eruptive material sends out veins into the surrounding rocks which are sometimes considerably