PRESIDENTIAL ADDRESS. 407 



probabilities of radio-active heating. Yet, so far as we have any record left to 

 us, Hutton remains fundnmentally in the right. All modern rese.arch shows that 

 the schists and gneisses can he explained by caiises now in action. The vast 

 majority of schfsts were at one time normal sediments ; others were tuffs or 

 lavas ; but, whether originally sedimentary or igneous, they owe their present 

 characters to widely spread regional metamorphism. 



The Undermining and Weakening of the Foundations of the Crust. 



Is there, then, any reason to depart from Hutton's position as to the 

 recurring cycle of events in the history of continental land ? I think it must 

 be admitted that the isostatic balance was far more frequently disturbed in 

 what we may call Lower pre-Cambrian times than it has been in more recent 

 periods. Osmond Fisher '° has pointed out the possibility of local melting of 

 the substratum of the crust by convection-currents in a liquid layer, and the 

 consequent weakening of the mass above. The differences in composition of 

 various parts of the crust render them, moreover, susceptible to fusion in various 

 degrees, whatever may be the source of the heat by which they are attacked. 

 Local fusion must indeed be regarded as an important cause of crustal weaken- 

 ing. If we wish to study the nature of the process, it is reasonable to examine 

 regions that have at one time lain deep within the crust. Such regions are 

 provided bv the broad surfaces of Archfean rocks that were worn down through 

 continental decay before they sank beneath the Cambrian sea. 



It is well recogni.sed that an ancient continent, resembling in most of its 

 features the present ' Russian platform,' at one time stretched across the 

 northern hemisphere. Wlierever later deposits have been stripped from its 

 surface, from central Canada to the Urals, and probably far beyond, we find 

 that the older materials of this undulating continental platform consist largely 

 of intrusive igneous rocks. These, moreover, have frequently a gneissic 

 structure. Again and again, strongly banded gneisses occur, in which granitic 

 material, verging on aplite, alternates with sheets of hornblendic or biotitic 

 schist. The biotitic varieties can often be traced back into amphibolites. In 

 places, lumps of these amphibolites are seen, streaked out at their margins, and 

 providing a clear explanation of the dark bands throughout the gneiss." This 

 swallowing up of a mantle of basic material by a very different and highly 

 siliceous magma rising from below is, indeed, seen to be a world-wide feature, 

 wherever we find the lower crust-lavers brought up within reach of observation. 

 The tuffs and lavas of the Keewatin series have supplied the dark material in 

 Canada, and similar rocks have been worked iip into the gneisses of Galway. 

 Stockholm, and Helsinki. The frequency of amphibolite in these ancient 

 composite rocks is explained by the fact that this type of rock is the final term 

 of various metamorphic series. While manv lumps, for insta,nce, in the gneisses 

 of Donegal are residues of Dalriadian dolerites (epidiorites), others, rich in 

 garnet and green pyroxene, and often containing quartz, are derived from a 

 mixture of sediments in which limestone has been prevalent." During the 

 absorption and disappearance of these masses in the invading granite magma, the 



'« Phiiaica of the Enrth's Crvnt. ed. 2 (1889), p. 77. 



" Since the historic works of A. C. Lawson (for example, 'Report on Rainy 

 Lake Region,' Geol. Rurv. Canndn. Ann. Report for 1887, plates v. and vi.), 

 these features have been traced in many areas. Compare W. H. Collins, 

 ' Country between Lake Nipigon and Clav Lake. Ontario,' Geol. Surv. Conrrdcr, 

 Publication 10,59 (1909), p. 52; A. L. Hall, Presidential Address on the Bush- 

 veld, Complex, Proc. Oe.ol. Ror. ,<?. .ifrira, 1914, p. xxii ; P. A. Wagner on 

 Rhodesian gneisses, Tram. i7)id., vol. xvii. p. 39; and works cited in the next 

 reference. 



" See Michel Levy, ' Granite de Flamanville,' Dull, carte gpn?. France. 

 vol. v. (1893). p. 337; G. A. .L Cole, 'Metamorphic Rocks in E. Tyrone and 

 S. Donegal.' Trnn.'^. R. Irish. Acad., vol. xxxi. (1900), p. 460; O. Triistedt. 'Die 

 Erzlagerstatten von Pitkaranta,' Bull. Comm. geol. Finlnnde, No. 19 (1907), 

 PP. 72 and 92; F. D. Adams and A. E. Barlow, op. rif. (1910), pp. 25 and 97: 

 T". Ki-etschmer, ' Kalksilikatf else in der ITmgebung von Mahrisch-Schonberg.' 

 ■In/irl). I.-. J.-, qrol. Beichxansfalf . vol. Iviii. (1908), p. .568; he. kc. 



