Page:Popular Science Monthly Volume 18.djvu/802

782 formed a frothy-like mass which cools as a light, spongy, vesicular material, so by the explosions from a trachytic volcano, similar masses are formed and thrown out, well known as the useful pumice-stone. This variety of lava produces often a very ragged surface, much less durable to mechanical agents than that of the other class. Again, it is very light, often more so than water. These differences, of course, merge into one another, lavas often occurring that are not easy to classify; but for our purposes the extremes are more suitable of illustration. Also, the same volcano may at different periods have yielded successively each of the varieties of igneous matter. Vesuvius, for instance, has ejected materials of each of the classes, and many distinct varieties of the basic. Obviously the discordance of these physical characters must necessarily produce considerable distinction in the physical conformation of a volcanic region in general, and of the cone in particular. It may be our want of a thorough examination, but it is apparently the rule that dikes are much less common among the trachytic volcanoes than the basaltic, whereas, apparently the largest number of breached cones belong to the former, thus contradicting to some small extent Mr. Mallet's dike theory already referred to. Thus we see that all the solids so far derived from a volcano, lava, scoria, lapilli, ash, etc., are all mechanical modifications of the one molten rock. There is, however, another important factor of which we have not spoken, the so-called ejected blocks. These are nothing more than fragments of the solid rock walls of the volcanic chimney or vent. They, therefore, vary according to the rock through which the igneous outburst has occurred. Thus we find among the constituents of the Vesuvian slopes a great variety of such blocks, among which the beautiful minerals yielded by Somma are found. These may be roughly divided into three classes:

1. Limestone variously metamorphosed, derived from that like Castellamare, which dips under and forms the Vesuvian platform. These fragments are sometimes so altered, by the intense heat, pressure, and chemical action to which they have been subjected, that it is only by studying the intermediate varieties that their origin can be detected. It is these blocks that are richest in the Vesuvian minerals.

2. Calcareous mudstones containing late pleistocene fossils, these being in a very perfect condition, containing generally a great number of well-preserved leaves. This rock is curious, as being of apparently (though not real) volcanic origin, and containing marine fossils without submergence.

3. Trachytic and corresponding tufa, also basaltic tufa. These are also masses of highly micaceous feldspathic rocks, that probably are nothing more than the excessive metamorphosed condition of the first class.—Science Gossip.