^^18.] OK THE rORMAtlOK OP ROCKS. 339 



foot of the Alps, about twelve ov fifteen leagues from 

 Turin, a dark coloured Serpentine is gradually ciiang- 

 ing into a carbonate of magnesia; which may be traced 

 through its progress at every step, from the beginniiig 

 to the end of the process; and there is no visible agent, 

 as great prirtof the rock is evidently bclow^ the influence 

 of the weather. Perhaps some new light might be 

 throw n upon some of nature's agents by a close e^ami- 

 ^nation of such changes. 



When geological researches are [Partial, and confin- 

 ed to a small portion of the surface, it is probable that 

 sufficient allow ance is not always made for that slow 

 and imperceptible change, which takes place in the 

 structure and exleriial flppearance of the rocks, with- 

 out the aid of any of the known agents, but by a pro- 

 cess as yet unknown ; hot having come within the 

 sphere of our observations \ve are ignorant of the mode 

 wliich nature takes to produce such changes. 



«HDER II. 



I 



Containing those rocks which analogy might be dis- 

 posed to place in the Volcanic origin. 



Hornhlend rocks, both greenstones and sieuites, as 

 well as the unmixed Hornblends, resemble some spe- 

 cies of lava, nearer than they resemble any rock of 

 undisputed Xeptunijin origin ; but in their relative po- 

 sitions, and regularity of extensive stratification, they 

 are similar to those of the Neptunian origin, as well as 

 in having pyrites, and other metallic substances, dissc- 



