Geology and Mineralogy. 471 



3. On the 'Volcanic Phenomena of the Eruption of Krakatoa, 

 <xnd on the nature and distribution of the Volcanic Materials ; 

 by J. W. Judd, F.R.S., Pres. Geol. Soc. 56 pp. 4to, with 6 

 plates. From the Report of the Royal Society Commission. — The 

 great eruption of Krakatoa, its results, and the character of its 

 ejected materials, are here ably discussed by Professor Judd, with 

 illustrating plates, maps, and wood-cuts. After sketching the 

 earlier history of the volcano, the author comes to the precursor 

 events of the eruption of 1883, which began May 20th, and then 

 to the culmination in the paroxysmal eruption of August 26th and 

 a'Zth. Views and maps show the changes that took place at the 

 time in the island and in the seas about it. The cone of Krakatoa 

 was left between 2,000 and 3,000 feet high, and the crater more 

 than 1,000 feet deep. 



Professor Jucld's explanation of the eruption is this : that the 

 interruption of the regular escape of vapors and ejections that 

 was consequent on the chilling of the surface of the liquid lava by 

 inrushes of sea-water caused a check and then a rally of the pent- 

 up force of gases seeking escape; that the catastrophic outburst 

 was a direct consequence of this " check and rally " of the sub- 

 terranean forces. 



The rocks of the cone and the ejected materials are described 

 in derail, and illustrated by microscopic sections. The rock about 

 the crater is enstatite-dacite or enstatite-andesite. There are also 

 black porphyritic pitchstones. The small cone of Rakata, at the 

 base of Krakatoa, consists of basaltic lavas, which are in part 

 chrysolitic. The formation of the ejected pumice from the pitch- 

 stone, on which the author had previously thrown much light, is 

 the subject of further remarks and explanations. He speaks of 

 the glass as being in a state of strain, as is proved by its depolar- 

 izing light, and this is mentioned as the cause of its extreme brit- 

 tleness, in consequence of which it easily crumbles to the finest 

 dust. The rocks are shown to have varied much in liquidity, 

 although having little difference in mineral constitution ; and this 

 is attributed to the amount of water they contain. Professor 

 Judd further observes that with water present and therefore a 

 low fusion point, there is less chance of devitrification in a liquid 

 lava ; whereas without water and a consequent high fusion point, 

 crystallization of minerals or lapidification is likely to take place 

 in the mass as cooling makes progress ; and thus from lavas of the 

 same constitution, under the one difference of amount of water, 

 rocks of very unlike aspect may result. 



4. Microscopical Physiography of the Rock-making Minerals 

 and aid to the microscopical study of rocks, by H. Rosenbusch. 

 Translated and abridged for use in schools and colleges by Joseph 

 P. Iddings. 333 pp. 8vo, with 26 plates of microphotographs, 

 New York, 1888. (John Wiley & Sons). The thanks of our 

 English-speaking students of Petrography are due to Mr. Iddings 

 for the excellent manner in which he has presented to them the 

 work of Prof. Rosenbusch. The value of this work is much too 



