lOO 



NATURE 



[September 23, 1915 



certain that we possess no record of a sedimentary 

 type peculiar to the early stages in the formation of a 

 habitable crust. If such a type existed, it has been 

 lost to us through subsequent metamorphism, amount- 

 ing to the actual fusion and redistribution of its con- 

 stituents. The Grenville Series of North America 

 rests on a floor of granitoid rock, which is intrusive 

 in it, and which belongs to the oldest of various 

 irruptive grouos. The Grenville Series includes con- 

 glomerates, false-bedded quartzites, and a develop- 

 ment of limestone that is altogether exceptional for 

 pre-Cambrian times. In Finland,* sediments have 

 been traced down to the layer where their original 

 characters vanish in a general " migmatitic " ground. 

 Conglomerates and phyllites occur among them, and 

 near Tampere (Tammerfors) the seasonal stratification 

 is as well recorded in a Bottnian shale as it is in the 

 Pleistocene clays made famous in Sweden by De Geer. 

 Vein-gneiss (Adergneis) underlies these ancient 

 systems, and represents their destruction by the injec- 

 tion of granite from below. If we accept the hypo- 

 thesis of Chamberlin, Hutton's position becomes 

 strengthened by the postulation of an unfused 

 planetesimal crust, and the restriction _ of molten 

 masses and hydrothermal activity to the interior of a 

 consolidating globe. 



So far as wo have any record left to us, 

 Hutton remains fundamentally in the right. All 

 modern research shows that the schists and gneisses 

 can be explained by causes now in action. The vast 

 majority of schists were at one time normal sediments ; 

 others were tuffs or lavas ; but, whether originally 

 sedimentary or igneous, they owe their present char- 

 acters 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 fre- 

 quently disturbed in what we may call Lower pre- 

 Cambrian times than it has been in more recent 

 periods. Local fusion must be regarded as an impor- 

 tant cause of crustal weakening. 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 by the broad sur- 

 faces of Archaean rocks that were worn down through 

 continental decay before they sank beneath the Cam- 

 brian sea. 



It is well recognised that an ancient continent 

 at one time stretched across the northern hemi- 

 sphere. Wherever 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 amphibolltes. 

 In places, lumps of these amphibolltes are seen, 

 streaked out at their margins, and providing a clear 

 explanation of the dark bands throughout the gneiss.* 



5 J. J. Sederholm, " Ueber eine archaische SeHimentformation im 

 siidwestlich^n- Finland," BuH Comni. geol. Fiulande, No. 6 (iSqq), p. 215. 



6 Since the historic work* of A. C. I.awson (for example, " Rei>ort on 

 Rainy Lake Rpgion," Geol, Sum. Canada, Ann. Report for 1887, plates v. 

 and vi.), these features have been traced in many areas. Compare W. H. 

 Collins, " Coiintrv between Lake Nipl eon and Clay Lake, Ontario," Geol. 

 Surv. Canada, Publication 1050 (1909^, p. 51;; A. L. HjiU, Presidential 

 Address on the Bush veld Complex, Proc Geol. Soc, S. Africa. 1914, 



xxii ; P. A. Wagner on Rhodesian gneisses, Trans, ibid., vol. xvii., 

 . 39 ; and works cited in the nfxt refe'e-ce. 



NO. 2395, VOL. 96] 



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-layers 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 up 

 Into the gneisses of Galway, Stockholm, and Helsinki. 

 The frequency of amphibolite in these ancient com- 

 posite rocks is explained by the fact that this type of 

 rock is the final term of various metamorphic series. 

 While many lumps, for instance, in the gneisses of 

 Donegal are residues of Dalriadian dolerltes (epi- 

 dlorltes), others, rich in garnet and green pyroxene, 

 and often containing quartz, are derived from a mix- 

 ture of sediments In which limestone has been preva- 

 lent.' During the absorption and disappearance of 

 these masses in the invading granite magma, the 

 amphibole acquires potassium and breaks down Into 

 biotite, and blotite-gneisses result, which may extend 

 over hundreds of square miles. 



The details of such an igneous invasion are worthy 

 of careful study, since only In this way can we follow 

 out the progress of subcrustal fusion. We see the 

 highly metamorphosed material further attacked by 

 the great cauldrons under it, and becoming seamed 

 with Intersecting veins. Block after block has been 

 caught, as It were, In the act of foundering into the 

 depths. In the gradual absorption of these blocks, 

 and their penetration by insidious streaks of granite, 

 we see pictured on a few square yards of surface the 

 destruction of a continental floor. 



The Invasion of a "hard and brittle"* crust by an 

 attacking magma was finely described by Lawson m 

 1888. Lawson pointed out that the Laurentian 

 gneisses gave no evidence of having " yielded to pres- 

 sures and earth-stresses." The folding of the over- 

 lying series was prior to the solidification of the 

 gneisses, and occurred ® " while the latter were yet 

 in the form of probably a thick, viscid magma upon 

 which floated the slowly shrinking and crumpling 

 strata of the Coutchlching and Keewatin series. ... 

 Large portions of these rocks have very probably been 

 absorbed by fusion with the magma, for the Lauren- 

 tian rocks appear to have resulted from the fusion 

 not simply of the floor upon which the Coutchlching 

 and Keewatin rock first rested, whatever such floor 

 may have been, but, also, with it, of portions of those 

 series." 



The conversion of the lowest Archaean series, the 

 Coutchlching sediments, into crvstalllne schists is 

 attributed to thermal metamorphism. and to hot 

 vapours streaming from the molten floor. Lawson 

 realised the Importance of shattering in allowing a 

 magma to advance Into an overlying "brittle" series, 

 and he Is, so far as I know, the first observer to 

 develop In satisfying detail what is now known as 

 the stopinf? theory of igneous intrusion. 



James Hutton always had In mind the effect of 

 heat In "softening" lower layers of the crust. His 

 consolidation of strata by heat is preceded by a stage 

 of melting. Sederholm. i* while' referring back to 

 Hutton as the pioneer, shows how in the vein-gneiss 



7 See Michel Lrfvy, "Granite de Flamanville," Bttll. carte geol. France, 

 vol. V (189:?), p. 337 ; O. A. J. Cole, "Metamorphic Rocks in E. Tyrone 

 and S. Doneeal," Trans K. Irish Acad., vol. xxxi. (1000), p. 460; 

 O. Triistedf. " Die Erzlaeerstatten von Pitkaranta," Bull. Comm. geol. 

 Finlande, No. 19(1907), pp. 72 and 92; F. P. Adams and A. E. Barlow, 

 flfi. cit. (loiol, p-^. 25 and 97: " F. Kretschmer, " Kalksilikatfelse in der 

 UrrgebunE von Mahrisch-Schonbcrg," Jahrh. k k. geol. Reichsamtalt, 

 vol. Iviii. (1908), p. 563 ; etc. 



S A. C. Lawson, o^. cii., p 140. See also his revision of the area, Geol. 

 Surii. Canada, Memoir 40 (1Q13). 



9 Of>. cit. on Rainy Lake, p. 131. 



10 " Ueber eine archaische Sedlmentformation im sii ^westlichen Finland," 

 Bull. Comm. geol. Finlande. No. 6 (1809), p. 133 ; and " Ueber ptygma- 

 tir.che Falfungen," Ncues J ahrh.Jiir Min., Beilag* Band 36 (1913), p. 491. 



