34 



NATURE 



[Septemher 15, 1910 



Continental ice-sheet with summer blizzards sweeping 

 across it like those of the Antarctic tableland, while the 

 gneiss beneath tells of a molten magma cooling during 

 millions of years beneath miles of overlying rock. 



It is the meeting-place of the geological extremes, and 

 their contact marks the greatest of all discordances. 



One thing the clay and the gneiss have in common — 

 both were long neglected by geology ; the Pleistocene beds 

 because they were not rocks, but only "drifts," confused 

 and troublesome things, hiding the real rocks, the orderly 

 stratified formations ; the " basal complex " because its 

 schisls and gneisses were fossil-less, complex, and 

 mysterious products of the dim beginnings of a world 

 still " without form and void." The molten sphere, with 

 its slowly consolidating crust, belonged rather to the 

 astronomer than the geologist. 



Geology has, of course, long lost that attjtude, and now 

 finds some of its most seductive problems in these once 

 neglected extremes of the earth's history. Those who 

 distrust the " glacial nightmare " are now .very few in 

 number ; but there are still revered veterans, like Prof. 

 Rosenbusch, who speak of the .Archaean gneisses as parts 

 of the earth's Erstarruiigskruste, and who frame theories 

 of the earth's cooling and wrinkling in its hot and furious 

 youth. 



Over more than half of Canada the field geologist is 

 forced to occupy himself with both the Pleistocene and the 

 Archaean, since the two are almost everywhere together, 

 while the fossil-bearing beds of the vast intervening time 

 are absent. The seemingly unnatural conjunction is not 

 entirely without advantages, for the Pleistocene has 

 furnished the clue to certain very puzzling problems of the 

 .Archaean, as will be shown later. 



The geologists of the world have long known the broad 

 outlines of the Canadian .Arch.Tcan or pre-Cambrian area 

 through Suess's masterly portrayal of the " Canadian 

 Shield," and through Dana's account of the '* Y Forma- 

 tion," about which the North American Continent was 

 built up. 



It must be remembered, however, that, though most of 

 the territory has been roughly traversed by Bell, Tyrrell, 

 Low, and other explorers, only a few districFs in the 

 south have had their geology worked out in detail, because 

 of their valuable deposits of silver, nickel, and iron ores. 

 It is only in these districts, and comparatively recently, 

 that the succession of pre-Cambrian formations has been 

 determined with certainty. In the wide spaces of the 

 north only the most general relationships are known. 



It is intended to bring together here our knowledge of 

 the most ancient chapters in the history of North America 

 as disclosed by recent field work. 



Physiographic Features. 



In its physiography, the Canadian Shield shows the 

 features that might be expected from one of the oldest 

 and most stable land areas of the world. It was reduced 

 in very early times to a peneplain, but later was elevated, 

 permitting the rivers to begin a process of dissection. 

 This process had a recent interruption by the Pleistocene 

 Ice Age', which blocked many of the valleys with moraines 

 and gave rise to the most extensive tangle of lakes in the 

 world. Physiographically, as well as geologically, the 

 region shows a dramatic mingling of extreme youth with 

 extreme old age. 



The best account of this rejuvenated peneplain has been 

 given by Dr. A. W. G. Wilson,' who shows that the 

 gradients are very gentle, and suggests that two or more 

 facets can be distinguished as having slightly different 

 inclinations and as having been carved at different times. 

 Here it will be unnecessary to take the matter up except 

 in a general way. 



The peneplain has been unequally elevated, parts stand- 

 ing 3000 or 4000 feet above the sea, and other parts sink- 

 ing beneath its surface. Only at two marginal points can 

 the Archaean surface be said to rise as mountains — in the 

 Adirondacks, projecting south-east into the State of New 

 York, and in the Nachvak peninsula, just east of Ungava 

 Bav. 



To the south-west and south the shield sinks, almost 



' " The Laurentian Peneplain, "■/(>«»•. Geo/., vol. xl. No. 7, pp. 615-659. 



NO. 2133, VOL. 84] 



imperceptibly in many places, beneath the older Palaeozoic 

 rocks, and the same is true around the central depression 

 of Hudson Bay. Toward the south-east the shield breaks 

 off suddenly along the great fault of the Lower St. 

 f^avvrence, and apparently the precipitous north-east shore 

 of Labrador indicates faulting on even a larger scale. It 

 has been suggested that Greenland, the Highlands of 

 Scotland, Scandinavia, and Finland may have been parts 

 of a single great shield, now separated through the settling 

 down of the sea-bottoms. 



In detail, the region is full of variety of hill and valley, 

 waterfall, river, and lake ; but, on the whole, it is mono- 

 tonous to the ordinary traveller from the constant repeti- 

 tion of similar forms, since there are no real mountain 

 ranges and few outstanding " monadnock " hills to break 

 the sky-line. The sweep of horizon from every hilltop 

 seems horizontal, the summits around seldom rising more 

 than 200 or 300 feet above the valleys, and all reaching 

 nearly the same elevation. 



The geologist finds', however, that this impression of 

 general flatness is deceptive. In reality, the rock struc- 

 tures are usually more nearlv vertical than horizontal, as 

 in most Archican regions. The schistose rocks, which 

 form so much of the surface, commonly show dips of 

 more than 60°, so that it is clearly a mountain region 

 planed down to its foundations. The arrangement of 

 valleys, ridges, and hills generally follows more or less 

 closely these ancient rock forms. 



Geologiial .Structure. 



Until recently, most of the geological work done in this 

 northern territory has been track surveys following Indian 

 canoe routes. Here and there moraines or old lake 

 deposits hide the rocks for a space, but usually the geology 

 is admirably displayed as one's canoe threads the intricate 

 waterways of sprawling lakes spilling over from one 

 irregular basin into another. On entering a new district 

 there seems a hopeless confusion of pinkish gneiss and 

 grey-green schist, but presently orderly forms take shape 

 upon the map as the numberless bays and islands are 

 explored, and the ground plan of vanished mountain ranges 

 begins to show itself. Dr. Andrew C. Lawson, in his 

 brilliant study of the Lake-of-the-Woods and Rainy Lake 

 regions in 18S4 to 188S, first brought out distinctly the 

 relationships, and later work has added greatly to our 

 knowledge of these ancient structures. 



The typical arrangement is that of rounded or oval 

 batholiths of gneiss, or of granite merging at the edges 

 into gneiss, with schists dipping steeply away from them 

 on all sides. Where the batholiths approach one another 

 the green schists occupy narrow troughs between. .As 

 shown by Lawson, they are evidently the bottoms of 

 synclines nipped in by the rising areas of granite and 

 gneiss. Round these eruptive masses the schists have a 

 strike parallel to the edge of the gneiss, so that they do 

 not form ordinary synclines, but widen and narrow and 

 swing in curves to adjust themselves to the varying rela- 

 tions of the batholiths. The meshes of green schist are 

 often not complete, the curving ends feathering out to a 

 point. In such places erosion has eaten the surface down 

 below the bottom of the syncline. 



The batholiths in Western Ontario are of all sizes, from 

 a mile to sixty miles or more in diameter, and they are 

 commonly somewhat elongated from west to east or from 

 south-west to north-east. They do not always follow one 

 another in orderly succession, but may lie scattered 

 irregularly, almost like bubbles on foamy water. Yet on 

 the large scale one can recognise a general trend in the 

 direction of the longest axes of the batholiths, and the 

 average strike of the schist in the various regions lies 

 between 50° and 80° east of north, conforming to the 

 same direction. This general east-north-east trend of the 

 basement structures doubtless reveals the axial relations 

 of the Archaean mountain ranges. 



It is sometimes stated that the so-called V formation of 

 North America was made up of two ranges converging 

 toward the south, the easterly arm of the V parallel to 

 the .Appalachian mountains and the westerly one to the 

 Rocky Mountains. The structural arrangement just out- 

 lined does not confirm this view, but suggests irregularly 

 parallel chains, cutting the direction of the Rockies about 



