1 20 Volcanos. 



we understand him, the process is not exactly mechanical or 

 chemical, and yet more of the former than of the latter, since 

 the laminae are supposed to be split asunder, so as to become 

 inconceivably divided. Still we would take leave to ask, 

 are the portions, however minute they may be supposed to 

 be, chemically united to the steam, dissolved in it, or, are 

 they mechanically suspended in an atmosphere of that hot 

 aerial agent : or, under such circumstances, does the distinc- 

 tion between chemical and mechanical suspension become 

 evanescent ? — We are aware that the author speaks of the 

 process as being mechanical, but perhaps there is nothing 

 with which we are acquainted that is exactly analogous. 



Mr. Scrope mentions the superior heat of deep mines, as 

 favoring the opinion that there is a great internal heat in the 

 earth. If we mistake not, it has been ascertained that this 

 arises from the superior density of the air, on account of the 

 greater pressure, upon the same principle that the greater 

 tenuity of the air contributes to the coldness of the higher 

 regions of the atmosphere. When mines are occupied by 

 water, there is said to be no evidence of heat, but on the 

 contrary, the temperature is lower. The hypothesis of a 

 central heat does not therefore appear to derive support from 

 this source, although we think it would not be difficult to 

 prove that there are physical causes, with whose nature and 

 energy we have sufficient acquaintance, to justify us in be- 

 lieving that ignition may at any time be generated in the 

 bowels of the earth. 



" The author now proceeds to examine the laws which deter- 

 mine the disposition of volcanic products on the surface of the 

 globe. The simple cone is first considered, resulting from the 

 accumulation of fragments projected by a series of explosions 

 from a single aperture. Its figure is a truncated cone, contain- 

 ing a funnel-shaped cavity called the crater. The line in which 

 the inner and outward slopes meet is the ridge. Its regularity- 

 is liable to disturbance from many causes, such as the fissure-like 

 form of the vent, which usually gives an oblong figure to the 

 cone; the vicinity of other vents; violent prevailing winds; 

 and, above all, the subsequent emission of a current of lava 

 from the same orifice by which one side of the crater is broken 

 down. Examples of these, and other varieties of figure, are 

 given from Auvergne, Italy, &c. 



" The composition of the cone is next dwelt on, and the na- 

 ture of the fragments, which are either, 1. Sconce, or portions 



