4'53. 



VOLCANIC AGENCi'. 105 



strata have been more or less modified by heat, acting under 

 great pressure and at various depths ; and that the present 

 position and direction of the rocks composing the crust 

 of the globe, have been produced by the same cause.* 

 The Huttonian theory, indeed, offers a satisfactory expla- 

 nation of a great proportion of geological phenomena, and 

 enables us to solve many of the most difficult problems 

 in the science ; and it is but an act of justice to the memory 

 of its illustrious founder, and of his able illustrator, Pro- 

 fessor Playfair, to state that this theory, corrected and 

 elucidated by the light which modern discoveries have shed 

 upon the physical history of our planet, is that embraced 

 by the most distinguished modern geologists. 



53. Volcanic agency. — Of the activity and power of 

 the agent to which these remarks more immediately refer, 

 the currents of melted rocks, called lavas, ejected through 

 crevices and fissures of the earth, accompanied with evolu- 

 tions of heat and vapour, afford the most striking proofs ; 

 and the volcano, with its frequent concomitant the earth- 

 quake, have in all ages excited the terror and astonish- 

 ment of mankind. It would be foreign to .the design 

 of this discourse, to dwell upon the nature and causes of 

 volcanic action. Dr. Daubeny,f Mr. Scrope,J and others, 

 have published highly interesting treatises on the subject ; 

 and Mr. Lyell has given an admirable sketch of volcanic 

 phenomena. § I will only advert to the increased tempera- 

 ture of the earth in proportion as we descend from the 

 surface towards the interior, and the profound depths from 

 which thermal waters take their rise, as tending to support 

 the opinion, that volcanic eruptions are occasioned by causes 



* See Playfair's Illustrations of the Huttonian Theory, vol. i. p. 33, 

 et seq. Edin. 1822. 



f Daubeny's Lectures on Volcanoes, 1826. 

 % Scrope's Considerations on Volcanoes, 1825. 

 § Principles of Geology, vol. ii. 



