on American Geology, 105 



metamorphism of carboniferous strata in eastern New England 

 points to the former existence of great deposits of newer and over- 

 lying deposits, which were subsequently swept away. 



On the subject of igneous rocks and volcanic phenomena, Mr. 

 Hall insists upon the principles which we were, so far as we know 

 the first to point out, namely their connection with great accumula- 

 tions of sediment, and of active volcanos with the newer deposits. 

 We have elsewhere said : " the volcanic phenomena of the present 

 day appear, so far as are aware, to be confined to regions of newer 

 secondary and tertiary deposits, which we may suppose the 

 central heat to be still penetrating, (as shewn by Mr. Babbage,) a 

 process which has long since ceased in the palaeozoic regions." 

 To the accumulation of sediments then we referred both modern 

 volcanos 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 attention moreover to the fact that the greatest 

 outbursts of igneous rock in the various formations appear 

 to be in all cases connected 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 connection the traps with the palaeozoic sandstones of Lake 

 Superior, and with the mesozoic sandstones of Nova Scotia and 

 the Connecticut and Hudson valleys. 



It may sometimes happen that the displaced and liquified sub- 

 stratum will find vent, not along the line of greatest accumulation, 

 but along the outskirts of the basin. Thus in eastern Canada it 

 is not along the chain of the Notre Dame mountains, but on the 

 north-west side of it that we meet with the great outbursts of 

 trachyte and dolerite, whose composition and distribution we have 

 elsewhere described. (Keport of Geological Survey for 1858, and 

 Am. Jour. Science, (2) xxix. 285.) 



The North American continent, from the grand simplicity of its 

 geological structure and from the absence, over great areas, of the 

 more recent formations, off'ers peculiar facilities for the solution of 

 some of the great problems of geology ; and we cannot finish this 

 article without congratulating ourselves upon the great progress 

 in this direction which has been made within the last few years 

 by the labors of American geologists. 



Montreal, March 1, 1861. 



