THE 



GEOLOGICAL MAGAZINE, 



NEW SERIES. DECADE II. VOL. V. 



No. VIII.— AUGUST, 1878. 



o:R,ia-iZNr.A.Xj JLI^TIOXJES. 



I. — On the Fokm of Volcanos. 



By Professor John Milne, F.G.S., 

 Imperial College of Engineering, Yedo, Japan. 

 (PLATE IX.) 

 TnEOM the short notices which are to be found about volcanos in 

 JU treatises on geology, we might often be led into the belief that 

 they were structures so regular in contour that their form was 

 indicative of their nature. In certain cases this appears to be so 

 remarkably true, that I wish to add a few observations to those 

 which have already been written upon the subject. In other cases, 

 however, the form of a volcano does not appear to have any character- 

 istics which distinguish it from other mountains, as will be seen by 

 glancing through the series of views given by Humboldt of the 

 volcanos of South America. The mountains in Iceland are also 

 very rough. When travelling in that country, where I saw and 

 ascended many volcanos, I do not remember that there was any- 

 thing about their shape more than their steepness and general 

 ruggedness which particularly attracted my attention.' The wilder- 

 ness of form presented to us by such mountains as these is so 

 evidently the combined effect of many and varied causes, that it 

 would be vain to seek a simple explanation for the formation of the 

 whole. 



With other volcanos which have been built up according to the 

 formula of our text-books, that is, by the ejection and accumula- 

 tion of material round a central vent, the case is different. I will 

 endeavour to show that such mountains which, for the want of a 

 more accurate term, have been called conical, have a particular kind 

 of regularity which does not appear to have been hitherto noticed. 

 Conversely, I wish also to point out, that if this particular form is 

 observed, that from it, not only is it possible to state how the 

 mountain has been built up, but also from any variations in this 

 form to determine the presence and dimensions of an internal core. 

 For these purposes I shall take the mountains which I have had the 

 opportunity of observing since my residence in Japan. The first 

 point to be observed about many of these mountains is the regularity 



1 This, however, is a statement made from memory, for shortly after my return 

 to England, my Icelandic sketches and note-books were unfortunately lost. 



DECADE II. VOL. V.— NO. VIII. 22 



