143. Stalactites, Stalagmites, Travertine.-When water containing carbonate of lime drops from the top of a cavern, as the water evaporates, the lime is deposited in successive layers on the floor (stalagmite), while an icicle-like rod of the lime depends from the roof (a stalactite); the two sometimes meet, forming a pillar. Travertine is a porous limestone, formed from the same cause in calcareous springs and streams. A solid object

immersed in such water becomes incrusted with limestone, or petrified, as it is called. A similar deposit of silica, called silicious sinter, is formed round hot springs. Hot water charged with soda dissolves an excess of silica, which is deposited as the water cools.

144. Metamorphic Rocks.-Metamorphic rocks retain the marks of their original stratification, but have had their structure altered, and have become more or less crystalline.

145. Quartz-rock or Quartzite is sandstone in which the original quartz granules have been partially fused and blended. Where the original sandstone was largely mixed with clay-compounds, the result is a gray rock called Grauwacke.

Fig. 25.-Bedding (SS), Joints (JJ), and Cleavage (DD).

146. Clay-slate is a fine-grained rock, generally of a

F

grayish-blue colour. It splits into thin lamina; and this cleavage seldom coincides with the original stratification, but crosses it at all angles. It is thus distinct from the lamination of shale.

147. Schists.-There are many varieties of schistsmica-schist, talc-schist, &c., according to the predominant mineral. They all agree in being crystalline, and in having the constituent minerals arranged, not confusedly as in the igneous rocks, but in layers parallel to the original bedding. This particular texture is called

foliation.

148. In Gneiss, which consists of quartz, felspar, and mica, the foliation is well marked, but many beds of gneiss can be traced until all marks of bedding are lost; the rock becomes gradually more crystalline, and at last passes into a true granite, as if part of an aqueous rock had been completely fused in situ. Granite may thus in some cases result from metamorphism carried to a high degree.

149. Crystalline Limestone.-In this the traces of the original organisms are obliterated. Saccharoid marble is a fine variety, resembling loaf-sugar in texture. Dolomite is an altered limestone, in which carbonate of magnesia has partly replaced carbonate of lime.

CHEMICAL ELEMENTS OF WHICH THE EARTH'S CRUST IS CHIEFLY COMPOSED.

150. According to Roscoe, the bulk of the earth's solid crust is made up of only eight elements, and that in the following proportions of 100 parts by weight:

Oxygen, in addition to entering into rocks, makes up 8ths by weight of all the water on the globe, and nearly of the atmosphere. On the supposition, then, that the core of the earth is composed of the same materials as the crust, we may reckon that one-half the weight of our globe is oxygen.

DISPOSITION OF STRATA.

151. We must conceive the layers of sedimentary rocks to have been laid down at first horizontally, or nearly so. In most cases, however, the original disposition has been disturbed. The strata are found at all degrees of inclination or dip, and frequently bear marks of having been bent, dislocated, and contorted.

Fig. 26 gives an idea of what frequently occurs. Beds

Fig. 26.-Effects of Bending and Denudation.

The

of limestone are seen coming to the surface-cropping out-at 1, 1, 1, among beds of shale and sandstone. two outcrops at 7, 7, on the left are evidently portions of one and the same stratum; and to explain how they come to be so disposed, we have to conceive that, when originally disturbed, the strata extended in a bend or undulation, as indicated by the dotted lines; and that these portions were afterwards worn away by denudation.

152. Unconformability.-When one set of rocks is found resting on the upturned edges of a lower set, the former are said to be unconformable to the latter.

In

the accompanying section (fig. 27), a, a, represent beds of sandstone resting on the upturned edges of limestone,

Fig. 27.-Unconformability.

shale, and sandstone, l, s. This pictures an immense lapse of time. We must first think of the deposition of the lower strata, one after another, in a horizontal position; then came the time of disturbance, when they were crumpled up into great convolutions; next began the process of planing away the tops of the convolutions; and finally, after a long lapse, another series of depositions succeed, terminated by an upheaval. The agents by which all this crumpling up and planing away have been effected, will be considered afterwards.

INTERNAL HEAT OF THE GLOBE.

153. Heat increases with Depth.-At a certain distance below the surface of the earth there is everywhere a line where the heat of summer and the cold of winter

are unfelt. The depth of this stratum of constant temperature varies with the climate and the nature of the rocks, from 20 to 50 feet. Beyond this depth, the heat is found to increase pretty regularly. The rate varies in different places, but the average for the whole globe is about 1° F. for every 50 feet of descent. This result has been arrived at from the numerous observations made in mines, in boring for artesian wells, and similar operations; and we cannot be sure that this rate holds beyond the limit of the observations, which is not over 3000 feet. But assuming that it does, at a

depth of 12,000 feet, we should have the temperature of boiling water; and, at 50 miles, the heat would be more than sufficient to fuse the most refractory substances known.

154. The Interior not Fluid.-It does not follow that the interior of our earth is actually in a fluid state. It is believed that the pressure of the superincumbent masses is sufficient to keep the interior matter in the solid state, notwithstanding the regular increase of heat. Were it not so, the attraction of the sun and moon would cause tides on the internal molten ocean, and the outer crust would partake of the undulations; but this we know is not the case. But although the earth is mainly solid, it is thought probable that some way below the surface there is a stratum of a honey-combed structure having cavities containing molten matter; and it is to this cavernous region that we are to look for the explanation of volcanoes and earthquakes.

155. Volcanoes.-A volcano is a conical hill or mountain, composed of matter that has been thrown out (erupted) from the interior of the earth. In the top is a cup-shaped cavity, called the crater, through which the erupted matter issues. When a volcano is dormant (which is different from being extinct), there is nothing to be seen externally but a light cloud of vapour floating on the top. On looking down into the cavity, jets of steam are seen issuing from various points in the flat space at the bottom, and often cavities full of red-hot lava. But at irregular intervals outbursts of violent activity occur. These outbursts are preceded by well-known symptoms-subterranean rumblings, tremblings of the ground, and fitful jets of steam and other vapours.

Fig. 28 is an ideal section of a volcanic mountain in eruption, and will give a notion of what takes place.