VI. 



THE PROBABLE SEAT OF VOLCANIC 

 ACTION. 



(1869.) 



The following paper was published in the Geological Magazine for June, 1869, and 

 reprinted, with an additional paragraph, in the Am. Jour. Science, from which it is 

 here reproduced. It is, as will be seen, to some extent a reinforcement of the views 

 advanced in Papers I. and II. ; but, notwithstanding the repetitions involved, it has 

 been thought proper to reprint it entire for the sake of the context. In further eluci- 

 dation of the subject I have appended some extracts from a lecture given in April, 

 1869, before the American Geographical Society in New York, and published in its 

 Proceedings, in which the distribution of volcanic and plutonic phenomena are con- 

 sidered. 



THE igneous theory of the earth's crust, which supposes it to 

 have been at one time a fused mass, and to still retain in its 

 interior a great degree of heat, is now generally admitted. In 

 order to explain the origin of eruptive rocks, the phenomena of 

 volcanoes, and the movements of the earth's crust, all of which 

 are conceived by geologists to depend upon the internal heat of 

 the earth, three principal hypotheses have been put forward. 

 Of these the first supposes that in the cooling of the globe a solid 

 crust of no great thickness was formed, which rests upon the 

 still uncongealed nucleus. The second hypothesis, maintained 

 by Hopkins and by Poulett Scrope, supposes solidification to 

 have commenced at the centre of the liquid globe, and to have 

 advanced towards the circumference. Before the last portions 

 became solidified, there was produced, it is conceived, a condi- 

 tion of imperfect liquidity, preventing the sinking of the cooled 

 and heavier particles, and giving rise to a superficial crust, from 

 which solidification would proceed downwards. There would 

 thus be enclosed, between the inner and outer solid parts, a 



