TRUE VOLCANOES. 327 



1cm presents a great number of values which are at present 

 undetermined. Among these we have to mention the influ- 

 ence of an enormous pressure upon fusibility ; the different 

 conduction of heat by heterogeneous rocks ; the remarkable 

 enfeebling of conductibility with a great increase of tempera- 

 ture, treated of by Edward Forbes ; the unequal depth of the 

 oceanic basin ; and the local accidents in the connection and 

 nature of the fissures which lead down to the fluid interior! 

 If the greater vicinity of the upper limit of the fluid interior 

 in particular regions of the earth may explain the frequency 

 of volcanoes and the greater multiplicity of communication 

 between the depths and the atmosphere, this vicinity again 

 may depend either upon the relative average differences of 

 elevation of the sea-bottom and the continents, or upon the 

 unequal perpendicular depth at which the surface of the molt- 

 en fluid mass occurs, in various geographical longitudes and 

 latitudes. But where does such a surface commence ? Arc 

 there not intermediate degrees between perfect solidity and 

 perfect mobility of the parts? states of transition which 

 have frequently been referred to in the discussions relative to 

 the plasticity of some Plutonic and volcanic rocks which have 

 been elevated to the surface, and also with regard to the move- 

 ment of glaciers. Such intermediate states abstract them- 

 selves from mathematical considerations, just as much as the 

 condition of the so-called fluid interior under an enormous 

 pressure. If it be not even very probable that the tempera- 

 ture every where continues to increase with the depth in ar- 

 ithmetical progression, local intermediate disturbances may 

 also occur, for example, by subterranean basins (cavities in 

 the hard mass), which are from time to time partially filled 

 from below with fluid lava and vapors resting upon it.* 

 Even the immortal author of the Protogcca allows these cav. 

 ities to play a part in the theory of the diminishing central 

 heat : " Postremo credibile est contrahentem se refrigeratione 

 crustam bullas reliquisse, ingentes pro rei magnitudine id est 

 sub vastis fornicibus cavitates."^ The more improbable it is 



chof '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. 

 ( Wiirmelehre des Innern ^msers Erdkorpers, s. 473.) 



* Cosmos, vol. v., p. 162. See also with regard to the unequal dis- 

 tribution of the icy soil, and the depth at which it commences, inde- 

 pendently of geographical latitude, the remarkable observations of 

 Captain Franklin^Erman, Kuptfer, and especially of Middendorff (toe. 

 cit. sup., s. 42, 47 and 107). 



f Leibnitz in the Protogcca ; 4. 



