^7(1 American Association. 



SUB-DIVISION OF THE LAUKENTIAN ROCKS OF OANABA. 



Hitherto these ancient and highly metamorphic rocks have 

 been regarded hy geologists as an inextricable mass of confusion. 

 Sir W. E. Logan has set himself to unravelling their intricacies, 

 with a patient perseverance of which hardly any other geologist 

 is capable. A summary of his results was presented in the fol- 

 lowing manner : — 



" I have already indicated the probable separation of 

 the Laurentian rocks of Canada into two great groups : that 

 characterized by the presence of much lime and that without ; but 

 from recent investigation, the result of which has just been reported 

 to the Canadian Government, it appears to me almost certain that 

 the former of these two great groups will be capable of subdivision, 

 and that some of its bands of limestone, with their associated strata, 

 are of sufficient importance to be represented separately on the 

 map. Having followed out one of these bands of limestone through 

 all its windings, for a distance of eighty miles, the object of tlie 

 present paper is to exhibit to the Section its geographical distri- 

 bution, and the forms it presents in tlie physical structure of the 

 reo'ion w liich it characterises. What at first appear to be two bands 

 of these limestones, emerge from beneath the LoAver Siuriau series 

 in the toAvnship of Grenville, on the Ottawa, and run into the 

 interior parallel to one another, striking N. I^T. E. They are about 

 two miles separated from one another, and both, with the gneiss 

 between, dip in one direction, which is N.N. W, at angles varying 

 from about 50 to 70 degrees. Attaining the rear of the township, 

 a distance of about ten miles, the two bands unite, and are found 

 really to constitute but one, the thickness of w'hich, as far as I 

 can make it out, is from 500 to 1,000 feet. It is plain from this 

 distribution that the limestone is part of the out crop of an undu- 

 latino- sheet, the ridges of which have been worn down. But in 

 the horizontal section of an undulating surface, similar forms in the 

 distribution of the rim, may be derived from the anticlinal or syn- 

 clinal part of the undulation, and as the dips on the opposite sides 

 are both one way, it is a question to which part the area belongs. 

 Within a short distance of the eastern side of the Hmestone, in fact, 

 touching it in one place, an intrusive syenite makes its appearance 

 belonging to a mass which occupies about 30 square miles in the 

 townships of Grenville and Chatham, and runs to a point in 

 Wentworth. The intrusion of such a mass of igneous rock as this 

 can scarcely foil to have had a considerable effect in modifying the 



