804 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



which constituted the primeval ocean. From this, the remaining 

 atmosphere, and a superficial portion of the solid crust, all the 

 present order of terrestrial things have been evolved. 



The Central Heat. 

 The conclusions as to the solidity of the earth's center, deduced 

 from chemical and phj'sical considerations, are further supported 

 by the astronomical calculations of William Hopkins on the phe- 

 nomena of precession and nutation, those of Thompson on the 

 tides, and those of Pratt on the pressure of mountain masses on 

 the earth's surface, all of which conduce to the conclusion that the 

 earth, if not solid to the center, must have a solid crust 2,000 miles 

 or more in thickness. The heat of the mass beneath the cooled 

 surface is still nearly that as Mhich the matter congealed ; the loss 

 of heat by radiation now proceeds very slowly. 



Action of Internal Heat. 

 This heat slowly conducted upward, alters, crystallizes, and 

 metamorphises the sedimentary rocks, and with the aid of water 

 everywhere diffused, gives rise to a soft and pasty layer upon 

 which the solid upper strata repose. From this softened layer 

 have come the eruptive and scotic rocks, which are portions of 

 these, unlike sedimentary beds displaced and forced upward 

 through fissures. Many of these beds are such mixed composition 

 as to yield, by their fusion, great volumes of various gases which 

 finding vent, together with the fused rocks or lavas, give rise to 

 volcanic phenomena. 



VOLCANOS. 



Volanic eruptions thus have their seat, not in the original 

 nucleus, which is solid, but in the deeply-buried strata heated by 

 this nucleus, and are no more than pustules or cutaneous eruptions 

 of the earth's surface. 



Active volcanos are only met with in or near regions where the 

 accumulations of sedimentary rocks has been great in comparatively 

 recent geological periods ; that is to say, where there are great 

 thicknesses of newer secondary or tertiary rocks. Hence their 

 absence from Eastern America ; and their occurrence along its 

 western coast. As by the chemical action of heat the deeply buried 

 strata are exhausted, volcanos become- extinct. 



