164 



KNOWLEDGE 



[Dkc. 23, 1881. 



purple l>loom, wliicli nclvfrtises to tlic birds tlio presence of 

 the sweet Juice within. The whoh^ process obviously aims 

 at concealing the fruit and rendcrinj; it unpalatable while 

 the seeds are immature, and at making it conspicuous as 

 woll a.s plea-sant the moment the .seeds are ripe for dis- 

 persion. Hence we are justilied in conchiding that the 

 develn].ment of the grape is due to the long selective action 

 of fruit-eating birds. Originally, no doubt, the primitive 

 ancestral vine produced smalh-r and harder seed-ves.sels, 

 which were probably green when young, and brown when 

 ready to fall upon the ground. But some "of them hap- 

 pened to show a tendency towards producing larger and 

 juicier fruits, and these were constantly favoured by the 

 unconscious friendliness of the neighbouring birds. Tlie 

 colour and the sweetness would soon follow, as they have 

 followed a thousand times over in the development of eacli 

 separate edible fruit. A grape, in short, viewed from the 

 standpoint of the vine itself, is merely a cunning device 

 for ensuring the assistance of birds or mammals in dis- 

 persing the little, nut-like seeds of which man takes, as a 

 rule, such scanty notice. 



STUDIES OF VOLCANIC ACTION. 



By G. F. Rodwell. 

 PART II. 



OF late years the microscopic study of eruptive rocks 

 has revealed many facts of importance. The most 

 opaque black lavas and basalts are seen to be maiiily made 

 up of colourless transparent crystals, when a slice less than 

 a hunflredth of an inch in thickness is placed under the 

 microscope. Polarised light enables us to distinguish the 

 nature of the crystals, and the angles can be readily 

 measured. Zirkel, Rosenbusch, and Rutlcy have done 

 much to promote this branch of petrology. One of 

 the most recent and beautiful works on the subject 

 is the " Min6ralogie Micrographique " of MM. Fouqu^ 

 and Michel Levy, which contains more than fifty 

 coloured quarto plates of rock sections seen under the 

 microscope, usually by polarised light. The rocks are, for 

 the most part, eruptive. A peculiarity of thi; book is an 

 ingenious plan for recognising in a moment the diflerent 

 mineral constituents of the section ; for this purpose each 

 plate is covered by a loose piece of transparent paper upon 

 which an uncoloured outline of the engraving is drawn, 

 and each mineral has its own number placed within the 

 outline. The same number is used throughout the book to 

 designate the same mineral. 



The fronti.spiece of Prof. Judd's "Volcanoes" shows six 

 beautiful sections of eruptive rocks, in which the passage 

 from the perfectly glassy to the highly-crystalline structure 

 is strikingly ilkistrat*'d. The first specimen is that of a 

 volcanic glass or obsidian, a lava which was rapidly cooled 

 from a condition of comphsto fluidity, and which shows 

 nebulous patches scattered through a glassy base. A very 

 high power re\eals that these patches are composed of 

 minute crystals, culled microlith^ or cri/atallili's ; and we 

 are forcibly reminded of the resolution of the heavenly 

 nebuliP into thickly-clustered stars by tlio microscope's 

 twin-sister. Sometimes the microliths are built up into 

 genn-like forms within the ground mass of the lava ; and 

 again, as the crystalline forces come more and more into 

 play, the microliths form radial groups about definite 

 centres, and thus build up globular masses, called " sphajru- 

 lites." Crystals are made up of microliths grouped aboiit 

 certain axes, and a completely amorphous glassy lava may 



and thus giving the molecvdar forces time to act in the 

 gi'Ouping of the microliths. Most lavas, when viewed 

 under the microscope, e.xhiVjit a glassy pa.ste or ground- 

 ma.ss, containing microliths, among which distinct crystals 

 are distributed. Or again, when lavas consolidate at a 

 gi-eat depth lieneath the surface, the ground-ma.ss is made 

 up of small crystals, through which larger crystals are dis- 

 tributed. And, finally, we arrive at the granitic structure, 

 in which the rock is completely made up of large crystals 

 without any ground-mass. One and the same rock may 

 exist in each of these fonns, according as it has been cooled 

 slowly or rapidly, at a great depth beneath the surface, or 

 near to it. Thus, while basalt represents the lava form of 

 a volcanic rock, yahbro is its granitic or crystallised form, 

 and tachijhjtn its glas.sy or obsidian form. They all have the 

 same ultimate chemical composition. 



The larger crystals probably separated from the amor- 

 phous masses beneath the volcano, and were carried up to 

 the surface bj- the fluid material forming the ground mass 

 of the lava. Crystals frequently furnish abundant evidence 

 of ha%ing been formed under enoimous pressure. When 

 examined by high powers of a microscope the crystals of 

 granitic rocks are sometimes seen to contain cavities filled 

 with liquid or gas, or with two liquids and a gas. Some- 

 times the liquid is water, sometimes a hydrocarlx)u like 

 the mineral oils which are found in abundance in deep- 

 seated rocks in various parts of the world. Not unfre- 

 quently the inclosed substance is liquefied carbonic acid, a 

 gas which requires a pressure of nearly 600 lb. on the 

 square inch to liquefy it at the freezing point of water, and 

 a much higher pressure at the temperature which exists a 

 short distance beneath the surface of the earth. We hav» 

 tlius every reason for believing that the crystals of a rock 

 have been formed in deep recesses in the earth, while the 

 ground mass has solidified at the surface. 



Of recent volcanic outbursts, there can be no doubt 

 that the eruption of Etna in May, 1870, was the most 

 considerable. It was studied very fully by Professor 

 Silvestri, who considers that it was the fulfilment of the 

 abortive attempt made by the volcano in 1874. On the 

 ■29th of August of that year a rift opened on the N.E. 

 side of the mountain between the crater and Mojo, and 

 thirty-fi\e monticules were tlirown up along its course, 

 with one large crateriform mound from which lava was 

 discharged. After seven hours of activity the dynamic 

 forces suddenly decreased, and in two days all the efl'ects 

 ceased, but the rift remained open, and earthquakes were 

 common in the vicinity. Silvestri then predicted that 

 when the next eruption occurred the rift woidd prove 

 the line of least resistance, and that lava would flow 

 from it, and craters be opened along its course, 

 and this prediction was completely verified in the 

 eruption of 1879. The fissure then extended itself 

 to a length of six miles, passing tlirough the great 

 crater. Eight eruptive mouths opened on the south 

 side of the mountmn, and discharged a small amount of 

 lava ; but the lava presently found an exit at a lower level 

 on the north side, and on May 28 Silvestri obseived a gi-eat 

 column of smoke, soon followed by the gloomy leaden tint 

 observed during an eclipse, and by showers of volcanic sand. 

 In ten minutes he collected more than two pounds of this 

 sand in an inverted umbrella. He then approached nearer, 

 to the scene of action, and, when about 6,200 ft. above the 

 sea, he heard loud dt'tonations and experi(>nced considerable 

 oscillations of the soil. As he approached the great rift, he 

 noticed three new cratei-s near Monte Nero, from which 

 issued dense clouds of steam. From one of them lava flowed 

 which formed a considerable stream, reaching to a distance 



