DIVISION OF ROCKS INTO HURONIAN AND LAURENTIAN. 439 



may not be the true formula of cinnabar, because, even if such be the 

 case, the peculiar character of the mineral may be accounted for by 

 the not improbable assumption, that the mercury is present in some 

 allotropic condition, essentially different from the normal state of mer- 

 cury as known to us in its isolated aspect. 



In the much-studied division of the silicates, we have a further proof 

 of the really indefinite nature of our present formulae, and consequently 

 of the uncertain value of the groups founded on this consideration. 

 For example : A1 2 3 replaces SiO 3 in certain augites and hornblendes. 

 Also, most probably, in staurolite, sillimanite, &c, and perhaps to a 

 certain extent in some spinels and sapphires. On the other hand it is 

 now universally allowed, that in the generality of silicates the formulae 

 are often greatly simplified by placing the A1 2 3 among the monatomic 

 bases, FeO, MgO, CaO, &c, — a fact brought out very prominently by 

 Professor Dana in the last edition of his System of Mineralogy, and 

 in other publications. Hence, if A1 2 3 sometimes replace SiO 3 , and 

 sometimes replace the bases RO, there is no reason why SiO 3 should 

 not also replace the latter in certain proportions. This granted, our 

 present formulae may be modified to almost any extent, and sub-groups 

 thus obtained to suit all cases. 



From these and other analogous considerations — such as will readily 

 suggest themselves to all who have made the investigation of minerals 

 their study — I think we may fairly admit that crystalline form and 

 atomic composition are not alone sufficient for the foundation of a 

 truly philosophic and satisfactory classification. Important as we may 

 allow these characters to be, they are not all-important. The general 

 aspect of the mineral, as indicating allotropic relations, its conditions 

 of occurrence, and other characters, must also be allowed a certain 

 value in the elaboration of at least our secondary groups. 



ON THE DIVISION OF THE AZOIC ROCKS OF CANADA 

 INTO HURONIAN AND LAURENTIAN. 



BY SIR WILLIAM E. LOGAN, F. R. S., 



DIRECTOR OP THE GEOLOGICAL SURVEY OF CANADA. 



Read before the American Association for the advancement of 

 Science, at Montreal, August \3th, 1857. 



The Subsilurian Azoic rocks of Canada occupy an area of nearly 

 a quarter of a million of square miles. Independent of their stratifica- 



