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ical Geography (1877); Outlines of Field Geology (1879); Geological Sketches at Home and Abroad (1882); Text-Book of Geology (1884); Class-Book of Geology (1886); Memoir of Sir A. C. Ramsey (1894); Ancient Volcanoes of Britain (1897). He was knighted in 1891.
VOLCANOES AND EARTHQUAKES.
It may seem at first as if it were hopeless that man should ever know anything about the earth's interior. In walking and moving over the surface of the earth we are like flies walking over a great hill. All that can be seen from the top of the highest mountain to the bottom of the deepest mine is not more in comparison than the mere varnish on the outside of a school-globe. And yet a good deal can be learnt as to what takes place within the earth. Here and there, in different countries, there are places where communication exists between the interior and the surface, and it is from such places that much of our information on this subject is derived. Volcanoes are among the most important of the channels of communication with the interior.
Let us suppose that we were to visit one of these volcanoes just before what is called an “ eruption.” As we approach it, we see a conical mountain, seemingly with its top cut off. From this truncated summit a white cloud rises. As we watch it we notice that it rises out of the top of the mountain, even though there are no clouds to be seen anywhere else. Ascending from the vegetation of the lower grounds, we find the slopes to consist partly of loose stones and ashes, partly of rough black sheets of rock, like the slag of an iron-furnace. At last we reach the summit'; and there what seemed a level top, is seen to be in reality a great basin, with steep walls descending into the depth of the mountain. We creep to the top of this basin, and look down into it. Far below the base of the rough red and yellow cliffs which form it's sides, lies a pool of some liquid glowing with a white heat, though covered for the most part with a black crust, like that seen on the outside of the mountain during the ascent. From this fiery pool jets of the red-hot liquid are jerked out every now and then; stones and dust are cast up into the air, and fall back again; and clouds of steam ascend from the same source, and form the uprising cloud which is seen from a great distance hanging over the mountain.
This caldron-shaped hollow on the summit of the mountain is the "crater." The intensely heated liquid in the sputtering, boiling pool at its bottom is melted rock or "lava." And the fragmentary materials ---- ashes, dust, cinders, and stones thrown out, are torn from the hardened sides and bottom of the crater by the violence of the explosion with which the gases and steam escape. The hot air and steam, and the melted mass at the bottom of the crater, show that there must be some source of intense heat underneath; and as the heat has been coming out for hundreds or even thousands of years, it must exist there in great abundance.
Volcanoes mark the position of some of the holes or orifices whereby heated materials from the inside of the earth are thrown up to the surface. They occur in all quarters of the globe. In Europe, besides Mount Vesuvius, which has been more or less active since it was formed, Etna, Stromboli, and other smaller volcanoes, occur in the basin of the Mediterranean; while far to the northwest some active volcanoes rise amid the snows and glaciers of Iceland. In America a chain of huge volcanoes stretches down the range of mountains which rises from the western margin of the continent. In Asia they are thickly grouped together in Java and some of the surrounding islands; and stretch thence through Japan and the Aleutian Isles to the extremity of North America. Thus the Pacific Ocean is girdled all round with volcanoes.
Since these openings into the interior of the earth are so numerous over the surface, we may conclude that this interior is intensely hot. But we have other proofs of this internal heat. In many countries hot-springs rise to the surface. It is known too that in all countries the heat increases as we descend into the earth. The deeper a mine the warmer are the rocks and air at its bottom. If the heat continues to increase in the same proportion, the rocks must be red-hot at no great distance beneath us.
It is not merely by volcanoes and hot-springs that the internal heat of the earth affects the surface. The solid ground is made to tremble, or is rent asunder, or upheaved or let down. These shakings of the ground, or earthquakes, when they are at their worst, crack the ground open, throw down trees and buildings, and bury hundreds of thousands of people in the ruins. Earthquakes are most common in or near those countries where active volcanoes exist.
Some parts of the land are slowly rising out of the sea. Rocks which used always to be covered by the tides, come to be wholly beyond their limits; while others, which used never to be seen at all, begin one by one to show their heads above water. On the other hand, some tracts are slowly sinking. Piles, sea-walls and other old landmarks on the beach are one after another enveloped by the sea as it encroaches further and higher on the land. These movements, whether in an upward or downward direction, are likewise due in some way to the internal heat.
When we reflect upon these various changes, we see that through the agency of their internal heat land is preserved upon the face of the earth. If rain and frost, rivers, glaciers, and the sea were to go on wearing down the surface of the land continually without any counterbalancing kind of action, the land would necessarily in the end disappear — and indeed would have disappeared long ago. But owing to the pushing out of some parts of the earth's surface by the movements of the heated materials inside, portions of the land are raised to a higher level while parts of the bed of the sea are actually upheaved so as to form land. This kind of elevation has happened many times in all quarters of the globe. Most of our hills and valleys are formed of rocks which were originally laid down on the bottom of the sea, and have been subsequently raised into land.-Physical Geography