TRUE VOLCANOES. 439 



the volcano of Jorullo. We are indebted for these remark- 

 able geosnostic accounts from the Mantschurei to the in- 

 dustry of W. P. Wassiljew (Q-eog. Bote, 1855, Heft, v, s. 31) 

 and to an essay by M. Semenow (the learned translator of 

 Carl Hitter's great work on Geology) in the 17th vol. of the 

 Proceedings of the Imperial Russian Geographical Society. 



In the course of the investigations into the geographical 

 distribution of volcanoes, and their frequent occurrence on 

 islands and sea-coasts, that is to say, on the margins of con- 

 tinental elevations, the probable great inequality in the 

 depth to which the crust of the earth has hitherto been 

 penetrated has also been frequently brought under con- 

 sideration. One is disposed to believe that the surface of 

 the internal molten mass of the earth's body lies nearest to 

 those points at which the volcanoes have burst forth. But 

 as it may be conceived that there are many intermediate 

 degrees of consistency in the solidifying mass, it is difficult 

 to form a clear idea of any such surface of the molten 

 matter ; if a change in the comprehensive capacity of the 

 external firm and already solidified shell, be supposed to 

 be the chief cause ot all the subversions, fissures, upheavals 

 and basin-like depressions. If we might be allowed to 

 determine what is called the thickness of the earth's crust 

 in an arithmetical ratio deduced from experiments drawn 

 from. Artesian wells, and from the fusion-point of granite, 

 that is to say, by taking equal geothermal degrees of 

 depth, 49 we should find it to be 20 ^ geographical miles 

 or 3-y^th of the Polar diameter. 60 But the influences of 



extremely careful, the highest water-shedding mountain-chain is that 

 on which is situated* the Karakorum pass (18,304 feet) which, stretching 

 from south-east to north-west, lies parallel to the opposite southerly 

 portion of the Himalaya (to the west of Dhawalagiri). The rivers 

 Yarkland and Karakasch, which form a part of the great water system 

 of the Tarim and Lake Lop, rise on the north-eastern slope of the 

 Karakorum chain. From this region of water-springs the travellers 

 arrived by way of Kissilkorum and the hot springs (120 F.) at the 

 small mountain lake of Kiuk-kiul, on the chain of the Kuen-liin 

 which stretches east and west (Report Xo. viii, Agra, 1857, p. 6). 



4%J Cosmos, vol. i, pp. 26, 167; see above, pp. 34 38. 



50 Arago (Astron. Populaire, t. iii, p. 248) adopts nearly the same 

 thickness of the earth's crust, namely, 40,000 metres, or about 22 

 miles; Elie de Beaumont (Systemes de Montagncs, t. iii, p. 1237), cal- 

 culates the thickness at about more. The oldest calculation is that 



