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NATURE 



[May 9, 19 1 8 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of NaTure. No notice is 

 taken of anonymous communications.] 



"Bread-crust" Volcanic Bombs. 



It is easy to identify the writer of the friendly 

 notice of Dr. Tempest Anderson's "Volcanic Studies" 

 (second series) in Nature of April iS, but, notwith- 

 standing his high authority, 1 must adhere to my 

 explanation of "bread-crust" bombs (p. 42), viz. that 

 the cracks are results, not of internal expansion, but of 

 contraction. 1 had the opportunity of studying a large 

 example near the crater of Vulcano, with others of 

 smaller size in the Lipari Islands, and brought away 

 a "hand-specimen," now in the Sedgwick Museum. 

 The former has a compact and rather glassy "rind" 

 about an inch thick, with an interior full of small 

 vesicles. The cracks vary much in size, and the walls 

 of the shallower converge (are rifts, in the strict 

 sense of the term) ; they appear to be analogous with 

 the cracks in septaria, etc., and I do not see how the 

 formation of numerous vesicles filled witli steam is to 

 increase the volume of the "core" within the "rind," 

 for the process is not comparable with one of effer- 

 vescence. Simple contraction of the crust seems in- 

 adequate, since these cracks, so far as I have seen, 

 do not occur in lumps which are homogeneous 

 throughout. 



In regard to admitting into the book some photo- 

 graphs which Dr. Anderson had already used as illus- 

 trations, I determined to risk the criticism, because I 

 knew them to be those which he preferred; so that 

 if I had excluded them I should have had to select 

 from the less valuable group, and I was anxious to 

 obtain a representative series. T. G. Bonney. 



It is certainly not without diffidence that any 

 British geologist will venture to question Prof. 

 Donney's interpretations of volcanic phenomena, but 

 in this case he is clearly in opposition not only to 

 received opinion, but also to well-established facts. 



A "bread-crust" bomb has a thin, compact rind 

 broken up into polygonal areas separated by cracks. 

 The' interior is usually highly vesicular, and even 

 pumiceous. An important point is that the cracks in 

 the crust frequently gape as if they had been opened 

 out, and into them the spongy matter of the interior 

 has sometimes risen up in such a manner as to sug- 

 gest that expansion has taken place after the crust 

 solidified. 



Prof. Lacroix, in his monograph on "La Montagne 

 Pelee" (p. 523), has given an excellent account of 

 them. "The concentric arrangement of these bombs 

 and the structure of their crust, different from that of 

 their interior, must be explained in the following 

 manner. When a portion of the magma at a very 

 high temperature is projected in a pasty condition the 

 surface rapidly cools, expelling the gases which it 

 contains. Thus the glassy crust is formed ; this 

 occupies a smaller volume than when molten, and 

 cracks from contraction, but at the same time the 

 centre of the bomb, cooling more slowly beneath the 

 protecting crust, gives off its gases only gradually. 

 As the solidifying glass becomes more viscous these 

 gases occasion the production of vesicles of varying 

 size, which increase the volume of the bomb. By 

 the conflict between the contraction of the periphery 

 and the expansion of the centre, ' lips ' are pro- 

 duced, and the fragments of the carapace are dis- 

 NO. 2532, VOL. lOl] 



placed as shown in plate xxiii." On p. 522 he ex- 

 plains the term "lips" as signifying open cracks in 

 "bread-crust" bombs, with the edges more or less- 

 everted like the leaves of a book. 



Prof. Mercalli, in his text-book of vulcanology, one 

 of the best and most recent (" Volcani Attivi," p. no), 

 gives practically the same explanation, and states that 

 these bombs are sometimes called ."bombe gonfiati " 

 (bombs that have swelled). Mercalli does not share 

 Prof. Bonney 's unwillingness to admit that the molten 

 material in bombs may effervesce, for he describes 

 "exploding bombs" that are ruptured with violence by 

 the expulsion of the gases in the magma, and cites 

 as authorities Prof. Ricco and Sir William Hamilton. 



The name was originally given by Prof. Johnston- 

 Lavis (Nature, vol. xxxix., p. no, and Proc. Geol. 

 Assoc, vol. xi., 1890, p. 392). He states that "expan- 

 sion causes cracking of the hardened crust, and in some 

 cases protrusion through the crust." His explanation 

 refers to the bombs of Vulcano (to which Ptof. 

 Bonnev's description also refers), and was accepted by 

 Prof. "Hobbs {Zeits. Dent. Geol. GeselL, vol. xlv., 

 p. 579) and by Prof. Bergeat ("Die Aeolische Inseln," 

 p. 185), both of whom know the island well, and have 

 made careful study of the petrology of these bombs. 



References might be multiplied if that were desir- 

 able, but sufficient has been said to prove that among 

 English, French, American, Italian, and German geo- 

 logists who have had every opportunity of observing 

 the facts, the explanation adopted by the reviewer is 

 generally regarded as the only satisfactory one. 



Recovery of Speech through Excitement. 



Public attention has recently been directed to several 

 remarkable examples of recovery of speech by shell- 

 shocked soldiers as the result of unexpected excite- 

 ment. Your readers may be interested to learn that a 

 very remarkable instance of the loosening of the tongue 

 occurred several thousand years ago, namely, in the 

 case of the afflicted son of Croesus, King of Lydia. 

 Cyrus, the Persian, besieged and took Sardis 548 B.C., 

 and Herodotus, writing approximately one hundred 

 years after the event, tells us that " when the town 

 was taken one of the Persians was just going to kill 

 Croesus, not knowing who he was. Croesus saw the 

 man coming, but under the pressure of his affliction 

 did not care to avoid the blow, not minding whether 

 or no he died beneath the stroke. Then this son of 

 his, who was voiceless, beholding the Persian as he 

 rushed towards Croesus, in an agony of his fear and 

 grief burst into speech, and said, ' Man, do not kill 

 Croesus.' This was the first time that he had ever 

 spoken a word, but afterwards he retained the power 

 of speech for the remainder of his life." (Herodotus, 

 book i., chap. Ixxxv., translated bv Rawlinson.) 



J. Newton Friend. 



London, May 6.. 



THE PROMOTION OF POST-GRADUATE 

 WORK AND RESEARCH. 



THE Senate of the University of London has 

 had under consideration proposals which have 

 emanated from. the Conference of Canadian Uni- 

 versities held in May, 191 6, the Conference of 

 Universities held on May 18, 1917, and the 

 American Association of University Professors. 

 The object in view is to encourage post-graduate 

 work and research and to contrive some means 

 of strengthening the ties between the universities 

 of Britain and her dependencies and those of the 



