Metamorphic Rocks.

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those regions of the earth where the strata have been subject to enormous lateral pressure. The chief agents of metamorphism thus appear to be great heat with water and immense lateral pressure. The chief changes of structure effected by metamorphic action are crystallization and foliation. As examples of metamorphic rocks we may mention marble, quartzite, slate, gneiss, and the schists.

Some

Crystalline Limestone, or statuary marble, is only limestone that has been altered by fusion so as to give it a granular texture like loaf sugar. marbles are coloured by mineral matter, and all are capable of being polished. Quartzite is sandstone that has been hardened by partial fusion, so that the grains of sand have run together, and can no longer be obtained separate on breaking the mass.

Slate is shale that has been hardened and otherwise changed by intense lateral pressure, and is composed mainly of aluminium silicate. In speaking of shale we described it as a hardened clay that splits into laminæ or thin leaves parallel to the layers of deposition. This shale was formed by film after film of fine sediment having been placed one on another, and this compacted by pressure from above, so that the films or laminæ acquired a tendency to separate one from another like the leaves of a book. The shale has undergone further alteration by great lateral pressure, causing it to split or cleave into leaves in a direction different from the bedding; and such altered shale is called slate. Cleavage, or slaty cleavage, is the name given to the tendency found in some rocks to split into layers or flakes more or less perpendicular to the original layers of deposition; while lamination (Lat. lamina, a leaf) is the name given to the arrangement in thin layers parallel to the bedding. Shale has a laminated structure, but slate shows cleavage.

Rocks

Gneiss. Gneiss is composed of the same minerals as granite, viz. quartz, felspar, and mica, but the minerals are arranged in layers or bands. in which the minerals are arranged in layers are said to be foliated, and this foliated arrangement of the minerals is the great peculiarity of gneiss. It occurs among the oldest known rocks of the earth's crust. Here is a drawing of a piece of gneiss, showing the foliated arrangement of the component minerals. The light layers consist of granular felspar, with here and there a little mica and quartz. The dark bands indicate layers of black mica intermingled with grains of grey quartz. the dark layers.

FIG. 130.-Gneiss.

The term schist (Gr. schizo, to split) is applied to any metamorphic rock that actually splits into folia or leaves not parallel to the original bedding. Those schists that have had their constituent minerals re-crystallized and re-arranged are called crystalline schists, and are named after their predominating mineral. Thus mica-schist is a crystalline metamorphic rock consisting of irregular wavy layers of mica and quartz.

142. Density of the Earth.-Astronomers have been able to calculate the weight of the earth, and have shown that it is about 5 times heavier than a globe of the same size composed of water. The mean density or specific gravity of the materials that form the crust is little less than 3. The average density of the earth is thus about twice the density of the rocks forming its crust. We are, therefore, forced to conclude that the interior portions of the earth are specifically heavier-i.e. of greater density than the outside crust. The explanation of this increase of weight in the materials of the earth's interior is a question still in dispute. In consequence of the enormous pressure exerted on the rocks in the interior by the rocks overlying them, some assert that these deeper rocks, composed of the same kind of materials as those in the crust, will be compressed or squeezed into so much less space, that this increase of density is easily accounted for, and that the increase of density would be still greater were it not for the expansive force of the heat in the earth's interior. Others doubt this great compressibility of solid substances, and believe that the interior portions of the globe are composed of heavier and different materials than those near the surface. These materials are thought to consist largely of metallic elements, such as iron, nickel, and cobalt, either free or forming alloys with one another. It has been pointed out that in this respect the earth's interior would be similar in composition to those pieces of matter flying through space which sometimes fall on the earth as meteorites.

Interior of the Earth.

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143. Internal Temperature of the Earth.-A thermometer on the surface of the earth, or buried only a few inches below the surface, shows variations of temperature depending on the time of the day and the season of the year. But these variations do not penetrate far into the crust. At a distance below the surface of about fifty feet in Britain there is a stratum of invariable temperature, this constant temperature being usually nearly the same as the average temperature of the surface. The depth at which this constant temperature is found varies somewhat with the climate. Below this depth the temperature increases, so that at the bottom of a deep mine the miners usually take off their clothes. The rate of increase varies for different places, being on the average 1° F. for every 60 feet of descent, or 88° F. for every mile. These facts have been ascertained by taking the temperature in mines, borings, and deep wells. In the Rose Bridge Colliery, near Wigan, a shaft has been sunk to the depth of 2445 feet, and the temperature at the bottom is about 94° F., the average temperature at the surface being 49° F. This gives an increase of about 1° F. for every 54 feet. Water rises from an artesian well near Paris from a depth of 1800 feet, and this water has a temperature of 82° F., being about 29° more than the average temperature. Other proofs of the high temperature of the interior of the earth are found in the existence of volcanoes sending out molten masses of rock, and in geysers and other hot springs discharging large volumes of heated water. These hot springs occur most frequently in the neighbourhood of volcanoes, but they are also found in other districts. There is one at Bath, the waters of which have a temperature of 120° F.

144. Interior not Fluid.-But though there is thus a high temperature existing in the earth's crust, and though there is a more or less regular increase of temperature with depth, it does not follow that the interior of the earth is in a liquid state. If the rate of increase (1° F. for every 60 feet) continues to depths far below those observed, it is certainly true at a depth of about two miles the rocks would be as hot as boiling water, while at a depth of forty or fifty miles the heat would be sufficiently great to melt any rock if at the surface. But at such a depth the pressure would be very great, and under increased pressure the melting-point of solid bodies is raised. They require more room when melted than when solid, and if the pressure prevents them from getting that space they cannot melt. We thus see that even if the temperature at great depths be above the fusingpoint of the rocks, they may be kept in the solid state by intense pressure. But we are not certain that the temperature at great depths does follow the regular law of increase observed in the rocks of the crust. In some places

the increase of temperature is much greater than the average given above, and in some places much less. It varies from 1° F. for every 20 feet of depth to 1° F. for every 100 feet. It has been noticed that the rate of increase is very high in places that have been recently the seat of volcanic activity. It may be pointed out that the calculations of astronomers show that the observed motions of the earth are inconsistent with a fluid interior. The most probable conclusion, therefore, is that the earth is mainly solid throughout, but that liquid spaces or cavities are formed by diminution of pressure or the inflow of water, both of which might reduce the fusingpoint of the rocks, and from these spaces volcanoes derive the material which they eject.

145. Definition of a Volcano.-It is not easy to give a satisfactory definition of a volcano, but we can easily avoid.

some of the mistakes often committed, and on which we shall afterwards remark. Here are examples of more or less accurate and complete definitions which will serve our purpose, and which will be better understood shortly. "A volcano is a more or less conical hill or mountain, usually truncated, communicating with the interior of the earth by a pipe or funnel, through which issue hot vapours and gases, and frequently loose fragmentary materials, and streams of molten rock."-Pref. J. Geikie. "A volcano is a more or less flat cone which is, or has been, connected by a channel with the depths of the earth, and which serves, or has before served, as an outlet for gaseous, solid, and glowing liquid masses."-Credner.

It will be noticed that in this last definition there is implied the fact that some volcanoes cease to send forth materials and become inactive. Hence the distinction between active and extinct volcanoes. "An active volcano may be defined as a passage or pipe which affords to deep-seated mineral matter in a state of fusion the means of transmission through the earth's crust, and of egress at its surface. A passive or extinct volcano is one in which this communication is obstructed, either by a plug of solidified lava, or by accumulations of fragmentary matter-a dissipation, temporary or permanent, of the eruptive energy permitting the solidification of the molten matter. Should an augmentation of the eruptive force occur, the plug will either be shattered and ejected in the form of lapilli and ashes, or re-melted and poured out as lava; but if it be unable to reopen the old passage, new vents may be produced, either within or without the lip of the crater."-Rutley. It will be noticed that in the definition of an active volcano, last given, no mention is made of a hill or mountain. No doubt this is because the essential and most important part of the volcano is the pipe or tube through which the ejected materials are sent, and also because the hills or mountains, when present, are built up out of the materials sent out, and are thus the results, and not the causes, of the volcanic activity. In the definition of an extinct volcano we must also notice that though no volcanic action of any kind is now taking place, yet it is possible that activity may again burst forth. Before A.D. 79 Vesuvius might have been regarded as an extinct volcano, for the great crater called' Somma, which then occupied the summit, had never been known to be in activity. But in that year it came into sudden activity; half of the crater was blown away, a great eruption took place, and the new crater, the present Vesuvius, was formed within the old one. It has continued in activity at intervals ever since, new cones being frequently formed, or old ones altered in shape. In the Auvergne district of Central France many denuded cones of extinct volcanoes still exist. No record or tradition of any eruption, however, remains. Even when the cone has entirely disappeared, the neck or solidified plug of lava may still remain to fix the site of a volcanic vent. When a volcano only throws out steam and gases, it is said to be dormant, or in a quiescent state. Renewed activity may occur at any time. A dormant volcano usually forms only a small cloud of steam near its summit.

146. Signs of an Eruption. In some cases eruptions take place without any warning symptoms, but in other cases there are indications for various periods beforehand. Loud rumbling sounds are heard from the mountain, and as these increase in intensity shocks of earthquake follow the noises, and the quakings return at shorter and shorter intervals. These form one of the surest and most general signs of an eruption, especially when the volcano was previously quiet. The water in wells and springs sometimes ceases to flow. This is no doubt owing to the disappearing of the water into rents and fissures below. At other times springs issue forth in fresh places, or wells which were before pure become muddy. The sea, too, may be affected by the ground shocks, rising and sinking in an unnatural way. But all these signs may fail and the eruption come quite suddenly, as did that of Vesuvius in 1853.

147. Phenomena accompanying an Eruption. After the signs and warnings, or without them, as may be, the vapour from the crater at the top of the vent increases in volume while the lava ascends in the pipe or funnel, and the phenomena usually occur in the following order:

(1) The outbreak begins with a mighty shake of the mountain, and the highly expanded steam and gases burst forth, scattering in tremendous explosions minute fragments of the lava. These explosions are quickly repeated, and huge ball-like masses of steam are driven upwards towards the sky. If there is little wind these rise nearly straight up and then spread out to form a horizontal cloud, so that the appearance of the column and cloud is like that of an Italian pine-tree called the stone-pine. This pinetree column, consisting of steam, gases, and fine particles of volcanic dust, reflects the glowing redness of the masses of heated rock and molten lava in the crater below, producing the appearance of flames. That they are not real flames, but produced by the reflection of the lava glow in the crater on the millions of steam bubbles in the column, is evident from the fact that they show no fluttering motion, and are not driven out of their steadiness by any wind. The steam-cloud soon begins to condense, and a great downfall of rain occurs, sweeping the loose volcanic dust down the slopes of the mountain, and forming torrents of mud-lava that often do more destruction than the real lava stream. Lightning and thunder accompany such an eruption, the electricity being probably generated by intense friction of the steam and dust particles.

(2) Closely following and partly accompanying the outburst of steam and gases there is a great discharge of dust, ashes, and stones. These are derived partly from the lava in the funnel, and partly from the walls of the crater, and either fall back on the sides of the cone, increasing the size of the mountain, or into the crater, whence they are again ejected, each time being reduced to finer powder. Some of the very fine powder is carried to