HARDWICKE'S SCIENCE-GOSSIP. 



223 



line in section. It is again to Mr. Mallet * that credit 

 is due for the explanation of this somewhat anomalous 

 appearance. It is known that the ground under high 

 towers and other heavy structures is gradually com- 

 pressed by the immense superincumbent weight. At 

 the same time a corresponding elevation takes place 

 around the base of the structure. This is just what 

 occurs in a volcanic mountain. The immense pressure 

 of superposed material compresses, to a variable degree, 

 the subjacent rock according to its yielding power. 

 This will be greatest where the column of materials 

 is highest, that is to say, exactly under the crater 

 edge as at C, in fig. 136. This causes a correspond- 

 ing rim-like elevation around the base, or at the toe 

 of the cone as at C", in diagram 136. 



The materials which go to form the cone are the 

 subjects of our next consideration. 



Taking as our standpoint the old but useful division 

 of lavas into basaltic or basic, and trachytic or acidic, 

 let us look at the characters presented by these two 

 great classes of rocks. Basalt and its congeners are 

 generally heavy, compact, dark coloured, more or 

 less crystalline. Very rarely vitreous in structure, 

 and only in small patches. Excessively fluid in the 

 molten state, losing heat and fluidity slowly, and 

 then passing rapidly from the liquid to the solid state, 

 the liquid fragments of which, when ejected from the 

 crater, generally fall still plastic, and when cold form 

 an excessively ragged hard angular mass. The surface 

 or scoria of the lava stream also is hard, and not 

 easily broken, the main mass itself being very apt to 

 form the well-known columnar structure. On the 

 other hand, the trachytic or acidic lavas, when 

 molten, are very viscid, which condition increases 

 rapidly as it loses its heat, so that it flows very short 

 distances, often stopping midway down the steep 

 side of the cone, as in the island of Vulcano, or 

 forming a large boss-shaped mass around the vent.f 

 When cooled slowly it crystallises, but it is much 

 more liable to form a vitreous mass or obsidian than 

 the basaltic rocks, resulting probably from its high 

 percentage of silica. In fact, it behaves very much 

 like glass or slag in its physical transformations. As 

 on the surface of the glass pot is 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 as 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, this scoria and pumice 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 



* R. Mallet, F.R.S. "Hitherto unnoticed circumstances 

 affecting the piling up of Volcanic Cones," Proc. Geol. Soc. 

 Lond. p. 740. 



t P. Scrope, F.R.S. "Volcanoes," 1862. 



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 examination, but it is 

 apparently the rule that dykes are much less common 

 amongst 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 * dyke theory already 

 referred to. Thus we see that all the solids so far 

 derived from a volcano, lava, scoria, lapilli, ash, &c, 

 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 amongst the constituents of the Vesuvian slopes a 

 great variety of such blocks, amongst 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 pleisto- 

 cene 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 

 Felspathic rocks, that probably are nothing more 

 than the excessive metamorphosed condition of the 

 first class. 



Hawthorn Bloom. — I can corroborate the state- 

 ment of R. W. in the August number, regarding the 

 entire absence of bloom on the whitethorn in the 

 north of Ireland. I know of old trees that never 

 failed flowering, but this year were destitute of 

 florescence. It appears to me that the scarcity of 

 bloom has arisen from the wet season of last year, and 

 consequently the young wood was not sufficiently 

 matured. — S. B. 



* Proc. Geol. Soc. Lond. vol. xxxii. p. 478. 



