Interior of the Globe, 3d 



located portion is to be sought above or below, according as 

 the line of fault, traced downwards across the bed, inclines 

 outwards from, or inwards to, its plane. Thus if w, a bed in 

 the mass e, is cut off by the fault t dividing e from d, the miner 

 seeks for the prolongation of it downward, and will find it at n, 

 because the line of fault t inclines outward from the plane of 

 the bed. If, again, he had been working on n, he would have 

 sought the prolongation upward, because the angle inclines in 

 the opposite way. The explanation is, that the fluid matter 

 in the cavity R exerts a greater pressure on the masses ac egi^ 

 whose broad sides are downwards, than onb df /*, whose nar- 

 row sides are downwards, and in the general movement the 

 former are therefore pushed farther up than the latter. Or, 

 to express it in another form, the latter slip down till their 

 immersion in the fluid counterbalances the narrower surface 

 exposed to its pressure. This curious fact in mining has long 

 been observed ; and, so far as I know, it has hitherto baffled 

 the ingenuity of geologists to give even a plausible explanation 

 of it. 



The hypothesis of the fluid matter existing in isolated cavi- 

 ties at a moderate depth, enables us to explain some of the 

 phenomena of volcanoes by the operation of an agent which is 

 known to be always present, namely steam or watery vapour. 

 Professor Bischof of Bonn, in an elaborate and learned me- 

 moir,* calculates that steam, at its maximum elasticity, is ca- 

 pable of supporting a column of liquid lava 17 miles in height. 

 The depth at which the internal heat of the globe would suf- 

 fice to keep lava in a state of fusion, is estimated at 20 or SO 

 miles ; but the data are too imperfect to indicate the ratio of 

 increase with any certainty. The increase, too, may follow a 

 geometrical, instead of the arithmetical, ratio assumed ; and in 

 this case the depth will be much less. Besides, if there is an 

 excess of heat at the bottom of a reservoir, the matter may be 

 kept fluid by circulation, to a level much nearer the surface 

 than the supposed limit of fusing temperature. Watery va- 

 pour, also, may not only reach the cavity containing the fluid 

 matter, but may mingle with it through chinks in the volcanic 



* Edinburgh New Philosophical Journal, January and April 1839. 



