border or rim around immense areas of the older gneisses of Division A, 

 Botli B and C carry iron ores; those of B being finely crystalline, 

 while those of C are coarsely crystalline. If these sub divisions prove 

 to be correct, and the lowest or Trembling Lake gneiss belongs to 

 Division A, the upper gneisses and limestones of Division C would in 

 the Grenville region have, according to Sir William Logan's measure- 

 ments, a thickness of about 15,000 feet. 



In Bavaria the Laurentian has been found to have a thickness of 

 not less than 90,000 feet. 



In closing, let me say a few words on the various views which have 

 been put forward to explain the origin of this immense series of 

 crystalline rocks which we have just been considering, asking you to 

 bear in mind that here we leave the domain of fact and enter that of 

 hypothesis. As yet comparatively little is known of the great Archaic 

 System as a whole, and every new investigation yields rich and unex- 

 pected fruit. Although, as Dr. Kalkousky i-emarks, * it is very difficult 

 to make our observations on these rocks completely objective, yet that is 

 the aim which we should have in view, and we may then hope, after 

 long continued, cai^eful and thorough investigation, to be able to decide 

 with a high degree of probability as to the method in which this great 

 system of rocks originated. Of all the theories hitherto put forward to 

 account for the genesis of these crystalline rocks, no single one is in a 

 position to give a satisfactory answer to all questions regarding them, 

 and, as ISTauman well says, — the real mode in which these primitive 

 rocks were formed is still involved in such obscurity that they may 

 with perfect justice be termed cryptogenous rocks, t 



Speaking generally, there are at the present time three hypotheses 

 maintained by different schools . According to the first of these the 

 rocks in question are the first solidified crust of our planet. The foliated 

 structure and banding not being due, like the bedding in more recent 

 formations, to aqueous deposition, but to agencies at work in the cool- 

 ing of a fluid mass, the earth as is usually supposed having been formed 

 by the gradual condensation of a mass of fiery vapour. We know that 



* Ueber die Erforschung der archiiischea Formationen — Neues Jahrbuch fur 

 Mineralogie, etc., 1880, II p. 1. 



t Lehrbrich der Geognosie — Zweiter Band s. 155. 



