EARTHQUAKES. 167 



since the terrible earthquake of Riobamba, which, on the 

 4th ot February, 1797, destroyed upwards of 30,000 people in 

 a few minutes, 8 we nevertheless saw the formerly advancing 

 cone of the Moya 9 which rose out of the earth, and witnessed 

 the employment of this combustible substance for cooking in 

 che huts of the Indians. I might describe the results of alte- 

 rations of the ground from this catastrophe, which, although 

 on a larger scale, were exactly analogous to those presented 

 by the famous earthquake of Calabria (February 1783), 

 and were long considered to have been represented ia an 

 incorrect and exaggerated manner, because they could not 

 be explained in accordance with hastily formed theories. 



By carefully separating, as we have already indicated, the 

 investigation of that which gives the impulse to the vibra- 

 tion, from that of the nature and propagation of the waves 

 of commotion, we distinguish two classes of problems of very 

 unequal accessibility. The former, in the present state of 

 our knowledge, can lead to no generally satisfactory results, 

 as is the case with so many problems in which we wish to 

 ascend to primary causes. Nevertheless, whilst we are en- 

 deavouring to discover laws in that which is submitted to 

 actual observation, it is of great cosmical interest that we 

 should bear constantly in mind the various genetic explana- 

 tions which have hitherto been put forward as probable. 

 As with all vulcanicity, the greater part of these refer, 

 under various modifications, to the high temperature and 

 chemical nature of the fused interior of the earth ; one of the 

 most recent explanations of earthquakes in trachytic regions, 

 is the result of geognostic suppositions regarding the want of 

 cohesion in rocky masses raised by volcanic action. The fol- 

 lowing summary furnishes a more exact but very brief indi- 

 cation of the variety of views as to the nature of the first 

 je to the commotion : 



The nucleus of the earth is supposed to be in a state of 

 igneous fluidity, as the consequence of every planetary 

 process of formation from a gaseous material, by evolution 



8 I follow the statistical account communicated to me by the Corre- 

 gidoi- of Tacunga in 1802. It rose to a loss of 30,00034,000 people, 

 but some twenty years later the number of those killed immediately 

 was reduced by about one-third. 



9 CVwnuw, voL i, p. 209, Bohn's edition. 



