58 THE ORIGIN OF MOUNTAINS. V.] 



to be confined to regions of newer secondary and tertiary 

 deposits, which we may suppose the central heat to be still 

 penetrating (as shown by Mr. Babbage), a process which has 

 long since ceased in the palaeozoic regions." * To the accu- 

 mulation of sediments then we referred both modern volcanoes 

 and ancient plutonic rocks ; these latter, like lavas, we regard 

 in all cases as but altered and displaced sediments, for which 

 reason we have called them exotic rocks. (Am. Jour. Sci. (2), 

 XXX. 133.) Mr. Hall reiterates these views, and calls atten- 

 tion moreover to the fact that the greatest outbursts of igneous 

 rock in the various formations appear to be in all cases con- 

 nected with rapid accumulation over limited areas, causing 

 perhaps disruptions of the crust, through which the semi-fluid 

 stratum may have risen to the surface. He cites in this con- 

 nection the traps with the palaeozoic sandstones of Lake Supe- 

 rior, and with the mesozoic sandstones of Nova Scotia and the 

 Connecticut and Hudson Valleys. 



* Ante, pp. 9 and 17. 



