202 ON THE CHEMISTRY OF METAMORPHIC ROCKS. 



in other words, the proportion between the alkalies and the alu- 

 mina, 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 unmetamor- 

 phosed, and exposed to the action of infiltrating waters. Thus 

 for example, that portion of the Lower Silurian rocks in Canada 

 which became metamorphosed before the close of the palaeozoic 

 period, will have lost less of its soluble bases than the portion of the 

 same age which still remains in the form of unaltered shales and 

 sandstones. Of these again, such parts as remain undisturbed by 

 folds and dislocations, will retain a larger portion of bases than 

 those strata in which such disturbances have favored the forma- 

 tion of mineral springs ; which even now are active in removing 

 soluble matters from these rocks. The crystalline Lower Silurian 

 rocks in Canada may be compared with those of the older Lau- 

 rentian series on the one hand, and with the Upper Silurian or 

 Devonian on the other ; but when these are to be compared with 

 the crystalline strata of secondary or tertiary age in the Alps, it 

 cannot be determined whether the sediments of which these were 

 formed, (and which may be supposed, for illustration, to have been 

 directly derived from palaeozoic strata), existed up to the time of 

 their translation, in a condition similar to that of the altered, or 

 of the unaltered Lower Silurian rocks of Canada. The proportion 

 between the alkalies and the alumina in the argillaceous sedi- 

 ments of any given formation is not therefore in direct 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 proportionate to the 

 newness of the formation, it is evident that the chemical and min- 

 eralogical 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 ages. 



It will be evident that silicious deposits, and chemical precip- 

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

 may exist with similar characters in the geological formations of 

 any age ; not only forming beds apart, but mingled with the im- 

 permeable silico-aluminous sediments of mechanical origin. Inas- 

 much as the chemical agencies giving rise to these compounds 

 were then most active, they may be expected in greatest abund- 

 ance in the rocks of the earlier periods. In the case of the per- 



