SECTION III.—THE VOLCANIC ROCKS.
The volcanic rocks consist of materials ejected from volcanoes. They are, however, ejected in very different states; sometimes as dust, sand, angular fragments of rock, cinders, &c., and sometimes as lava streams. In some instances, the lava has so little fluidity that it accumulates in a dome-shaped mass over the orifice of eruption, and perhaps in a few instances it has been thrust upward in a solid state.
There are two principal varieties of lava, the trachytic, consisting mostly of felspar, and the basaltic, consisting of hornblende. When both kinds are products of the same eruption, the trachytic lava is thrown out first, and the basaltic last. The reason of this is, that felspar is lighter than hornblende, and probably rises to the surface of the lava mass at the volcanic focus, and the basaltic lava is therefore reserved till the trachytic has been thrown off.
These, like other rocks, have been produced at different epochs. There is, however, great difficulty in determining their age; There are some differences of structure and composition observed, in comparing the older and newer lavas; but the only method that can be relied on to determine their age is their relation to other rocks. When they occur between strata whose age is determined by imbedded fossils, they must be of intermediate age between the inferior and superior strata.
1. Modern Volcanic Rocks.—Some of the volcanic rocks are of modern origin, and are produced by volcanoes now active. The total amount of these, and of all the other volcanic rocks, is probably less than that of either of the other principal divisions of rocks; yet they form no inconsiderable part of the earth’s crust. The number of active volcanoes is not far from three hundred, and the number of eruptions annually is estimated at about twenty. In some cases, the lava consists of only a single stream, of but a few hundred yards in extent. It extends, however, not unfrequently twenty miles in length, and two or three hundred yards in breadth. The eruption of Mount Loa, on the island of Hawaii, in 1840, from the crater of Kilauea, covered an area of fifteen square miles to the depth of twelve feet; and another eruption of the same mountain, in 1843, covered an area of at least fifty square miles. The eruption in Iceland, in 1783, continued in almost incessant activity for a year, and sent off two streams in opposite directions, which reached a distance of fifty miles in one case, and of forty in the other, with a width varying from three to fifteen miles, and with an average depth of more than a hundred feet. The size of some of the volcanic mountains will also assist in forming an idea of the amount of volcanic rocks. Monte Nuovo, near Naples, which is a mile and a half in circumference and four hundred and forty feet high, was thrown up in a single day. Ætna, which is eleven thousand feet high, and eighty-seven miles in circumference at its base, has probably been produced wholly by its own eruptions. A large part of the chain of the Andes consists of volcanic rock, but the proportion we have not the means of estimating.
2. Tertiary Lavas.—There is another class of volcanic products, which are so situated with reference to the tertiary strata that they must be referred to that period. The principal localities of these lavas, so far as yet known, are Italy, Spain, Central France, Hungary, and Germany. They are also found in South America. Those of Central France have been studied with the most care. They occur in several groups, but they were the seats of volcanic activity during the same epoch, and formed parts of one extensive volcanic region. Each of these minor areas, embracing a circle of twenty or thirty miles in diameter, is covered with hills two or three thousand feet in height, which are composed entirely of volcanic products, like the cone of Ætna. On many of them there are perfectly-formed craters still remaining. Numerous streams of lava have flowed from these craters, some of which can now be traced, throughout their whole extent, with as much certainty as if they were eruptions of the present century. Some of the lavas have accumulated around the orifices of eruption, forming rounded, dome-shaped eminences. These lavas generally consist of trachyte, and have therefore a low specific gravity, and imperfect fluidity. The basaltic lavas have often spread out over broad areas, and, when they have been confined in valleys, have reached a distance of fifteen miles or more from their source. There still remain indications of a current of lava which was thirty miles long, six broad, and in a part of its course from four to six hundred feet deep. The above sketch ([Fig. 5]) will give some idea of the highly volcanic aspect which the district of Auvergne, in France, presents.
The unimpaired state of some of the cones and craters, and of the lava currents, would lead to the impression that these regions have been the theatre of intense volcanic action within a very recent period. But there is good reason to believe that this has not been the case. “The high antiquity of the most modern of these volcanoes is indeed sufficiently obvious. Had any of them been in a state of activity in the age of Julius Cæsar, that general, who encamped upon the plains of Auvergne and laid siege to its principal city, could hardly have failed to notice them.”