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XVII.— THE RELATIONSHIP OF THE STRUCTURE OF ROCKS 

 TO THE CONDITIONS OF THEIR FORMATION. By 

 H. J. JOHNSTON LA VIS, M.D. 



[Read, February 17, 1886.] 



Introduction. — Two great questions of Yulcanology, of which 

 our knowledge is still very limited, are in the first place the causes 

 that bring about the wide range in force and character of volcanic 

 activity ; and, secondly, the difference in structure and composition 

 of the resulting products. In the present Paper it is proposed to 

 discuss a portion only of these very complex questions. 



Those who have lived in a still active volcanic region, and have 

 gazed over the landmarks of former activity, and compared them 

 with those at present in progress, cannot but be struck by the evi- 

 dence afforded of the enormous disproportion in the exhibition of 

 volcanic energy from one time to another. Before 1631 the crater 

 of Yesuvius was clothed with trees, brushwood, and grass, where 

 goats were pastured, while the only sign of igneous action was 

 the presence of two small lakes of warm water occupying the 

 bottom of that depression. The quietness of this sylvan scene was 

 only broken by the twitter of birds, the shepherd's chant, or the 

 wind-rustled leaves. Let us compare this state of placidity, or still 

 more that in which no sign whatever existed of the endogenous 

 activity in the time of Spartacus, with those gigantic, prehistoric 

 eruptions which tore away 1,000 metres or more of the mountain 

 top, and hollowed out a cavity equal to a cone with a base diameter 

 of three kilometres and a height of 1400 to 1500 metres. Even 

 the Plinian eruption was an insignificant affair compared with its 

 four predecessors. Between these two extremes in the vital activity 

 of a volcano we have all stages of gradation. 



Yet the phenomena of Yesuvius bear the proportion of child's 

 play to a giant's exploits when we compare them with the catas- 

 trophe of Tomboro, Krakatoa, Cotopaxi, the Icelandic volcanoes, 

 and many others. Meditating on these facts can hardly fail to 

 awaken within us the inquiry as to the actuating cause of the 

 variability in functions, if we may so call them, of a volcano. 



SCIEN. FROC., R.D.S., VOL. V. PT. III. K 



