782 THE POPULAR SCIENCE MONTHLY. 



formed a frothy-like mass which cools as a light, spongy, vesicular ma- 

 terial, 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 dis- 

 tinction in the physical conformation of a volcanic region in general, 

 and of the cone in particular. It may be our want of a thorough ex- 

 amination, but it is apparently the rule that dikes are much less com- 

 mon among the trachytic volcanoes than the basaltic, whereas, appar- 

 ently 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 vol- 

 cano, 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 Yesuvian 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 Cas- 

 tellamare, 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 with- 

 out 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. 



* " Proc. Geol. Soc," London, vol. xxxii, p. 478. 



