34 THE DAWN OF LIFE. 



tional lists of their beds will be found in the Reports of the 

 Geological Burvey, and Dr. Hunt has discussed very fully 

 their chemical characters and metamorphism in his Chemical 

 and Geological Essays. The recent reports of Hitchcock on 

 New Hampshire, and Hayden on the Western Territories, 

 contain some new facts of interest. The former recognises 

 in the White Mountain region a series of gneisses and other 

 altered rocks of Lower Laurentian age, and, resting uncon- 

 formably on these, others corresponding to the Upper Lau- 

 rentian ; while above the latter are other pre-silurian formations 

 corresponding to the Huronian and probably to the Montalban 

 series of Hunt, These facts confirm Logan's results in Canada ; 

 and Hitchcock finds many reasons to believe in the existence 

 of life at the time of the deposition of these old rocks. 

 Eayden's report describes granitic and gneissose rocks, pro- 

 bably of Laurentian age, as appearing over great areas in 

 Colorado, Arizona, Utah, and Nevada — showing the existence 

 of this old metamorphic floor over vast regions of Western 

 America. 



The metamorphism of these .rocks does not imply any 

 change of their constituent elements, or interference with 

 their bedded arrangement. It consists in the alteration of the 

 sediments by merely molecular changes re-arranging their par- 

 ticles so as to render them crystalline, or by chemical reactions 

 producing new combinations of their elements. Experiment 

 shows that the action of heat, pressure, and waters containing 

 alkaline carbonates and silicates, would produce such changes. 

 The amount and character of change would depend on the 

 composition of the sediment, the heat applied, the substances 

 in solution in the water, and the lapse of time. (See Hunt's 

 Essays, p. 24.) 



