ON ITS EXTERIOR. VOLCANOES. 323 



the Caucasus, Abich, is inclined, from his own observa- 

 tions, to regard the great eruption of pumice near the 

 village of Tschegem, on the northern side of the central 

 chain of Elbouruz, as an effect of action from fissures 

 much more ancient than the elevation of that conical 

 mountain, from which it is at a considerable distance, 



If, according to the view which we have taken of vol- 

 canic activity, we regard the diminution of the earth's ori- 

 ginal temperature, by the radiation of its heat into space, 

 as the original cause, and the cooling and contraction 

 of the outer strata as the immediate effect, producing 

 fractures and foldings in the outer strata, in which 

 foldings some portions will be raised, and some portions 

 depressed ( 461 ), it will follow that the number of re- 

 cognisable volcanic frame-works (open cones and dome- 

 shaped elevations), upheaved over fissures, should be 

 regarded as the measure and evidence of the degree of 

 this activity in different regions. Such computations 

 have often been made in a very imperfect and unsatis- 

 factory manner, and sometimes mounts of eruption and 

 solfataras belonging to one and the same system, have 

 been enumerated as distinct volcanoes. The large 

 spaces in the interior of continents, which have hitherto 

 remained closed to all scientific examination, have not 

 been such great obstacles in the way of these inquiries 

 as may have been commonly supposed; as, generally 

 speaking, volcanoes are most often met with in islands, 

 and in regions near the coast. In a numerical inquiry, 

 which the state of our knowledge will not permit us to 

 render complete, much may still be gained by our 

 being able to assign an inferior limit, and to state, with 

 probably considerable approximation, the number of 



r '2 



