326 



COSMOS. 



the fundamental distinction of Central and Linear Volcanoes, 

 under one cosmical point of view. My own more recent, and, 

 probably for this reason, more complete enumeration, under- 

 taken in accordance with principles which I have already in- 

 dicated (p. 233 and 257), and therefore excluding unopened 

 bell-shaped mountains and mere eruptive cones, gives, as the 

 probable lower numerical limit {nombre limite inferieur), a result 

 which differs considerably from all previous 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 thosa 

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

 volcanoes are crowded together, and the reaction of the inte- 

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

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

 face? Whatever be the course adopted to determine the av- 

 erage thickness of the solid crust of the earth in its maximum : 

 whether it be the purely mathematical one which is present- 

 ed by theoretical astronomy,* or the simpler course, found- 

 ed upon the law of the increase of heat with depth and the 

 temperature of fusion of rocks, f still the solution of this prob- 



* 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, 

 p. 45-49. 



f Cosmos, vol. v., p. 38-40; Naumann, Geognosie, bd. i., p. 66-76; 

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

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

 oVun 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 123LV' "This low esti- 

 mate," as Gustav Rose observes, "is founded in an error. The tem- 

 perature of 2372°, which is given by Mitscherlich as the melting point 

 of granite ( Cosmos, vol. i., p. 25), 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 feldspar to form a vesicular glass ; the quartz be- 

 comes 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 de- 

 tection of quartz in rocks, in which its quantity is so small that it can 

 not be discovered with the naked eye ; for example, in the syenite of 

 Plauen, and in the diorite which we brought in 1829 from Alapajevvsk, 

 in the Ural. All rocks which contain no quartz, or any other miner- 

 als 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 neverthe- 

 less certainly capable of producing a temperature of 1231°." In Bis- 



