TRANSACTIONS OF SECTION C. G33 



scope, whicb may be included under the term ' dynamic metamorphism,' were 

 wholly unknown and unforeseen. 



In admitting that chemical, metamorphic, and uniformitarian hypotheses were 

 thus given, in turn, undue weight, it is not to be assumed that the advances made 

 under these hypotheses have been entirely lost ; it has been necessary only to retreat 

 in part in each instance, in order to fall again into the more direct road. 



In late years, modern microscopical and chemical methods of research have 

 been applied to the ancient crystalline schists of Canada — the older work has been 

 brought under review, and new districts have been entered upon with improved 

 weapons. Here, as in other parts of the world, investigations of the kind are 

 still in active progress ; finality has not been reached on many points, but the ex- 

 planation of others has been found. One advance which deserves special mention 

 is the recognition of the fact that a great part of the Huronian is essentially com- 

 posed of contemporaneous volcanic material, effusive or fragmental. This was 

 first clearly stated by Canadian geologists, but has only become generally admitted 

 by degrees, in opposition to prevalent theories of metamorphism and cosmic 

 chemistry. 



The first opportunity of studying these Archaean rocks in detail, under the new 

 conditions, fell to Dr. A. C. Lawson, then on the staff of the Canadian Survey, in 

 the vicinity of the Lake of the Woods and elsewhere to the west of Lake Superior. 

 In that part of the Protaxis, the Laurentian appears to be represented only by the 

 Fundamental Gneiss, and the Huronian, by a series to which a local name 

 (Keewatin) was appropriately given,' but which is now Icnown to differ in no 

 essential respect from many other developments of the same system. The 

 Huronian stands generally in compressed folds, and along the line of junction the 

 gneisses are related to it in the manner of an eruptive, penetrating its mass and 

 containing detached fragments from it. The same or very similar relations have 

 since been found to occur in many other places. 



Arguing from observations of the kind last mentioned, it was too hastily assumed 

 by some geologists that the Laurentian as a whole is essentially igneous, and later 

 in date than the Huronian. The conditions are, however, not such as to admit of 

 an unqualified belief of this kind, even in regard to the Fundamental Gneiss. We 

 may go so far as to assume that these rocks (occupying as they do much the 

 larger part of the entire Protaxis) constitute a great ' batholitic ' mass of material 

 at one time wholly fluent ; but even on this hypothesis some primitive floor must 

 have existed upon which the Huronian and the similarly circumstanced Grenville 

 Series were laid down, and no such enormous substitution can have obtained as to 

 result in the replacement of the whole of this floor by exotic material.- It seems 

 much more probable that but limited tracts of the Fundamental Gneiss have 

 passed into a fluent condition when at great depths in the earth's crust, and 

 various arguments may be adduced in favour of a belief that the observed lines of 

 contact might be those along which such fusion would be most likely to occur.* 

 Moreover, the Huronian in many and v/idely separated localities is found to con- 

 tain water-rounded fragments of syenitic, granitic and gneissic rocks, forming 

 conglomerates, which may often be observed to pass into schists, but still plainly 

 indicate that, in these places at least, materials not unlike those of the Funda- 

 mental Gneiss and its associates were at the surface and subject to denudation. 

 Such materials cannot be regarded as parts of any primeval superficial crust of the 

 earth in an original condition. They represent crystalline rocks formed at great 

 depths, and under conditions similar, at least, to those under which the Funda- 

 mental Gneiss was produced. They imply a great pre-Huroniau denudation, and 

 show that the Huronian must have been deposited unconformably either upon the 



' In the Archaean, local names are particularly useful, inasmuch as correlation 

 must proceed on lithological and stratigraphical data, more or less uncertain when 

 extended to wide areas, even in the case of the older and more homogeneous strata 

 of the earth's crust. 



' For analogous phenomena of much later date geologically, see Annual Report 

 Geological Survey of Canada, 1886, p. 11 B. 



' Hypotheses on this subject are well summarised by Van Hise. Annual Report 

 U.S. Geol. Survey, 1894-95, p. 749. 



