APPENDIX. 



Note I., page 50. 



The doctrine of the central igneous fluidity of the earth is generally ac- 

 cepted by geologists. The hypothesis of intense chemical action as the 

 cause of existing internal heat — a hypothesis first enunciated by Sir Hum- 

 phry Davy* — is, however, revived from time to time under some novel 

 modification. Dr. T. S. Hunt, while admitting the primordial incandes- 

 cence of our planet, has maintained, in a series of lectures before the Low- 

 ell Institute of Boston, that the solid crust is probably not less than 2000 

 miles in thickness, and envelops a solid nucleus, with a comparatively thin 

 belt of material between the two, which has been reduced to a soft and 

 pasty condition by the combined action of heat, water, and chemical affin- 

 ity. In his lecture on " Primeval Chemistry, " more recently delivered 

 before the American Institute in New York, he is reported as saying that 

 "the earth must have a crust several hundred miles in thickness;" that 

 "granite is in all cases a secondary rock, derived from sediments crys- 

 tallized through the agency of water and heat;" and that "the theory 

 which ascribes volcanic products to the supposed uncooled liquid centre 

 fails entirely to account for the great diversity in composition of these 

 products, all of which, wherever found, are represented in rocks of aque- 

 ous origin." 



Mr. N. S. Shaler has attempted to show, in an ingenious paper read be- 

 fore the Boston Society of Natural History (Proceedings, vol. xi. , p. 8), that 

 the solidification of the earth began at the centre and proceeded toward 

 the periphery — that finally solidification began at the periphery and pro- 

 ceeded toward the centre, leaving, within the era of recognizable geologi- 

 cal events, but an insignificant portion of the earth in its primordial fluid 

 state. 



* "Unless, indeed, Milton can be said to have first suggested it in the following 

 words : 



"The force 

 Of subterranean wind transports a hill 

 Torn from Pelorus, or the shattered side 

 Of thundering ^Etna, whose* combustible 

 And fueled entrails thence conceiving fire, 

 Sublimed with mineral fury, aid the winds, 

 And leave a singed bottom all involved 

 "With stench and smoke."— Paradise Lost, i., 230. 



T 



