64 THE HISTORY OF THE NORTH ATLANTIC 



basalts, affords a probability that the inner magma is in part 

 metallic, and possibly, that vast masses of unoxidised metals 

 exist in the central portion of the earth. 



(5) Where rents or fissures form in the upper crust, the 

 material of the lower crust is forced upward by the pressure 

 of the less supported portions of the former, giving rise to 

 volcanic phenomena either of an explosive or quiet character, 

 as may be determined by contact with water. The underlying 

 material may also be carried to the surface by the agency of 

 heated water, producing those quiet discharges which Hunt 

 has named crenitic. It is to be observed here that explosive 

 volcanic phenomena, and the formation of cones, are, as 

 Prestwich has well remarked, characteristic of an old and 

 thickened crust; quiet ejection from fissures and hydro- 

 thermal action may have been more common in earlier periods 

 and with a thinner over-crust. This is an important con- 

 sideration with reference to those earlier ages referred to in 

 chapter second. 



(6) The contraction of the earth's interior by cooling and 

 by the emission of material from below the over-crust, has 

 caused this crust to press downward, and therefore laterally, 

 and so to effect great bends, folds, and plications ; and these, 

 modified subsequently by surface denudation, and the piling 

 of sediments on portions of the crust, constitute mountain 

 chains and continental plateaus. As Hall long ago pointed 

 out, 1 such lines of folding have been produced more especially 

 where thick sediments had been laid down on the sea-bottom, 

 and where, in consequence, internal expansion of the crust had 

 occurred from heating below. Thus we have here another 

 apparent paradox, namely, that the elevations of the earth's 

 crust occur in the places where the greatest burden of de- 



1 Hall (American Association Address, 1857, subsequently republished, 

 with additions, as "Contributions to the Geological History of the American 

 Continent"), Mallet, Rogers, Dana, La Conte, etc. 



