III.] THE CHEMISTRY OF METAMOEPHIC EOCKS. 27' 



the mean composition of the argillaceous sediments of any 

 geological epoch, or, in other words, the proportion between 

 the alkalies and the alumina, will depend not only upon the 

 age of the formation, but upon the number of times which 

 its materials have been broken up, and the periods during 

 which they have remained unmetamorphosed, and exposed to 

 the action of infiltrating waters The proportion be- 

 tween the alkalies and the alumina in the argillaceous sedi- 

 ments of any given formation is not therefore in flirect relation 

 to its age ; but indicates the extent to which these sediments 

 have been subjected to the influences of water, carbonic acid, 

 and vegetation. If, however, it may be assumed that this 

 action, other things being equal, has on the whole been pro- 

 portionate to the newness of the formation, it is evident that 

 the chemical and mineralogical composition of different systems 

 of rocks must vary with their antiquity ; and it now remains 

 to find in their comparative study a guide to their respective 



It will be evident that silicious deposits and chemical pre- 

 cipitates, like the carbonates and silicates of lime and magnesia, 

 may exist with simi^r characters in the geological formations 

 of any age ; not only forming beds apart, but mingled with 

 the impermeable silico-aluminous sediments of mechanical ori- 

 gin. Inasmuch as the chemical agencies giving rise to these 

 compounds were then most active, they may be expected in 

 greatest abundance in the rocks of the earlier periods. In the 

 case of the permeable and more highly silicious class of sedi- 

 ments already noticed, whose chief elements are silica, alumina, 

 and alkalies, the deposits of different ages will be marked 

 chiefly by a progressive diminution in the amount of potash 

 and more especially of the soda which they contain. In the 

 oldest rocks the proportion of alkali will be nearly or quite 

 sufficient to form orthoclase and albite with the whole of the 

 alumina present ; but as the alkali diminishes, a portion of 

 the alumina will crystallize, on the metamorphism of the sedi- 

 ments, in the form of a potash-mica, such as muscovite or 

 margarodite. While the oxygen-ratio between the 'alumina 



