258 



NATURE 



\yuly 



21, 



possessor of a thoroughly methodical habit of mind, he 

 can scarcely hope to arrange all the facts as they 

 should be. 



After an introductory chapter on the general nature of 

 the inquiry instituted in the succeeding chapters, the 

 author discusses "the nature of volcanic action." To 

 illustrate this he takes the case of the ever-active volcano 

 Stromboli, first examined scientifically by Spallanzani in 

 1788, and by Mr. Judd in 1874. It is a conical mountain 

 rising 3090 feet above the sea, but the shore slopes to a 

 depth of nearly 600 fathoms, hence the real height of the 

 mountain from the bottom of the ocean exceeds 6000 feet. 

 On the upper side of the crater a spot exists from vifhich 

 it is possible to look down upon the floor of the crater, 

 and here may be seen apertures in which three classes of 

 action take place. From some' high-pressure steam is 

 emitted in loud puft's ; from others masses of molten lava 

 well out ; and in the third kind, a semi-liquid substance 

 may be seen heaving up and down. Sometimes it rises 

 as a kind of scum, swollen by the steam beneath it, and 

 at last a gigantic bubble of molten lava filled with steam 

 appears, and bursts ; the imprisoned steam then escapes, 

 carrying with it masses of the bubble high into the air. 

 The author considers that all volcanic phenomena depend 

 on these same conditions : (ci) cracks or apertures forming 

 communication between the surface and the interior of 

 the earth ; {b) highly heated matter beneath the surface ; 

 -and {c) imprisoned water. 



Animadverting on the comaion delusion that a volcano 

 is a burning mountain, and that sulphur is the com- 

 bustible, it is shown that sulphur is the result and not 

 the cause of volcanic action. Common constituents of 

 volcanic action are sulphurous acid and sulphuretted 

 hydrogen, and when these come into contact, according 

 to the author, " water and sulphuric acid are formed and 

 a certain quantity of sulphur is set free." It should be 

 understood however that if the gases are at all dry, as 

 they sometimes are in the Solfataras of Krisuvik and else- 

 where, water and sulphur are the sole products of the 

 decomposition, while if moist, sulphur, water, and penta- 

 thionic acid are the first result. 



If we examine the history of Vesuvius and other vol- 

 canic centres which have been known from a remote 

 antiquity, we are led to the following conclusions, as 

 regards the frequency of outbursts : — " (i) A long period 

 of quiescence is generally followed by an eruption which 

 is either of long duration, or of great violence. (2) A 

 long-continued, or very violent eruption is usually fol- 

 lowed by a prolonged period of repose. (3) Feeble and 

 short eruptions usually succeed one another at brief 

 intervals. (4) As a general rule, the violence of a great 

 eruption is inversely proportional to its duration." 



In the third chapter the author describes the products 

 of volcanic action. In the account of \'ulcano he has 

 omitted to mention the very remarkable substance lately 

 analysed by Prof Cossa of Turin, which contains no 

 fewer than se\-en non-metals and eight metals, combined 

 in the following forms : — Arsenious sulphide, selenium 

 sulphide, boric acid, ammonium chloride, lithium sul- 

 phate, thallium alum, cssium alum, rubidium alum, and 

 potassium alum. Bunsen's important division of all lavas 

 into " acid lavas" and "basic lavas" is accepted, and the 

 author admits an intermediate lava which contains from 



55 to 66 per cent, of silica. He divides lavas further into 

 five great groups : the Rhyolites, Trachytes, Andesites, 

 Phonolites, and Basalts ; the first being acid, the last 

 basic, and the three others intermediate. An interesting 

 account (illustrated by the frontispiece) is given of the 

 microscopic examination of thin sections of rock, and the 

 practicability of tracing by this means the passage from a 

 glassy to a crystalline lava. It is shown that volcanic 

 rocks having precisely the same chemical composition 

 differ considerably in texture according as they are cooled 

 slowly or rapidly. Thus gabbro, basalt, and trachyte are 

 respectively the crystalline, lava, and glassy forms of the 

 same substance. Some interesting details are given of 

 the liquid cavities found in certain crystals, and of their 

 contents. 



In the fourth chapter the distribution of materials 

 ejected from volcanic vents is discussed. In the account 

 of " Pele's Hair "■ — the long threads of lava blown out 

 by high-pressure steam in Hawaii — the reader may be 

 misled. The author speaks of it as "filamentous vol- 

 canic glass," and in the passage preceding it (p. 71) he is 

 evidently discussing " glassy lavas " and " pumice," which 

 have been ranged among the acid lavas. But on p. 94 

 the same lava of Kilauea is spoken of as a " basic lava," 

 although before described as a " molten glass," and pre- 

 sumably acid in character, that is, containing from 66 to 

 80 per cent, of silica. But basalt is a basic lava, and by 

 rapid cooling it may become a perfect glass, hence we 

 can understand how Pele's Hair may be described as 

 " filamentous glass,'' without belonging to the class of 

 acid lavas. 



In the following chapter an extremely interesting 

 account of the dissection of volcanoes by denudation is 

 given, and the subject is illustrated by some striking 

 examples, among which we may specially mention the 

 plan of the volcano of Mull in the Inner Hebrides. In a 

 past geological period this volcano was probably as large 

 as Etna. The Islaod of Skye is the basal wreck of 

 another volcano of Tertiary times. In the account of the 

 formation of mineral veins the author has not alluded to 

 Bunsen's surmise that the metallic copper found in the 

 palagonite tuff of the Faroe Islands was reduced by vol- 

 canic hydrogen from the chloride. 



The sixth chapter treats of the parasitic cones which 

 appear upon the flanks of great volcanoes, and herein we 

 notice one or two errors. Thus on p. 162 we read, "Among 

 the hundreds of parasitic cones which stud the flanks of 

 Etna, there are some which are nearly 800 feet in height." 

 There are however less than a hundred cones worthy 

 of the name, the rest are mere monticules, and of these 

 we believe there are over six hundred. Among the larger 

 cones Monte Minardo is the largest, and it is 750 feet in 

 height, but has undoubtedly been much higher. In Fig. 

 63, p. 163, the outline of Etna, as seen from the Val del 

 Bove, is wrongly described. The picture represents Etna 

 as seen from Bronte, the opposite side of the mountain to 

 the Val del Bove. It is taken from von Waltershausen's 

 " Atlas des ^tna," and appears in Mr. Scrope's book 

 on Volcanoes, in which it is also wrongly described. 

 Occasionally we meet with hasty writing, particularly 

 when the author is firmly convinced of his statement. 

 Such small defects are easily remedied in the second 

 edition. The following is an example of what we mean : 



