TRUE VOLCANOES. 347 



vious ones. It is an attempt to indicate the volcanoes 

 which have been active within the historical period. 



The question has been repeatedly raised whether in those 

 parts of the earth's surface, in which the greatest number 

 of volcanoes are crowded together, and the reaction of the 

 interior of the earth upon the hard (solid) crust manifests 

 the most activity, the fused part may not lie nearer to 

 the surface ? Whatever be the course adopted to determine 

 the average thickness of the solid crust of the earth in its 

 maximum : whether it be the purely mathematical' one 

 which is presented by theoretical astronomy 39 , or the simpler 

 course, founded upon the law of the increase of heat with 

 depth and the temperature of fusion of rocks 40 , still the 

 .solution of this problem presents a great number of values 

 which are at present undetermined. Amongst these we 



39 W. Hopkins, Researches on Physical Geology in the Phil. Transact, 

 for 1839, pt. ii, p, 311, for 1840, pt. i, p. 193, and for 1842, pt. i, p. 43 ; 

 also with regard to the necessary relations of stability of the external 

 surface; Theory of Volcanoes in the British Association Report for 1847, 

 pp. 4549. 



40 Cosmos, vol. v. pp. 35 37 ; Naumann, Geogncsie, Bd. i, pp. 66 76 ; 

 Bischof, Wdrmelehre, s. 382 ; Lyell, Principles of Geology, 1853, pp. 536 

 547 and 562. In the very interesting and instructive work, Sou- 

 renirs dun Naturaliste, by A. de Quatrefages, 1854, t. ii, p. 469, the 

 upper limit of the fused liquid strata, is brought up to the small depth 

 of 20 kilometres : " as most of the silicates fuse at 1231." " This low 

 estimate," as Gustav Rose observes, " is founded in an error. The 

 temperature of 2372, which is given by Mitscherlich as the melting 

 point of granite (Cosmos, vol. i, p. 26) is certainly the minimum that we 

 can admit. I have repeatedly had granite placed in the hottest parts of 

 a porcelain furnace, and it was always but imperfectly fused. The 

 mica alone fuses with the felspar to form a vesicular glass ; the quartz 

 becomes opaque, but does not fuse. This is the case with all rocks 

 which contain quartz ; and this means may even be made use of for 

 the detection of quartz in rocks, in which its quantity is so small that 

 it cannot be discovered with the naked eye, for example in the 

 syenite of Plauen, and in the diorite, which we brought in 1829 from 

 Alapajewsk in the Ural. All rocks which contain no quartz, or any 

 other minerals so rich in silica as granite, such as basalt for example, 

 fuse more readily than granite to form a perfect glass in the porcelain 

 furnace ; but not over the spirit lamp with a double current, which is 

 nevertheless certainly capable of producing a temperature of 1231." 

 In Bischof s remarkable experiments, on the fusion of a globule of 

 basalt, even this mineral appeared, from some hypothetical assumptions 

 to require a temperature 264 higher than the melting point of copper. 

 (Wdrmelekre des Jnnern unsers Erdkbrpcrs, s. 473). 



