PRESIDENTIAL ADDRESS. 409 



gneisses gave no evidence of having ' yielded to pressures and earth-stresses.' 

 The folding of the overlying series v^^as 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 Coutchiching and Keewatin series. . . . Large portions of these 

 rocks have very probably been absorbed by fusion with the magma, for the 

 Laurentian rocks appear to have resulted from the fusion not simply of the 

 floor upon which the Coutchiching and Keewatin rock first rested, whatever 

 such floor may have been, but, also, with it, of portions of those series.' 



The intense crumpling of the lower portion of the invaded series is not 

 correlated by Lawson with the approach of the invader. The conversion of the 

 lowest Archpean series, the iCoutchiching sediments, into crystalline schists is, 

 however, 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 stoping theory of igneous intrusion. J. G. Goodchild -' soon 

 afterwards described a striking example of rock-destruction by stoping and 

 assimilation in the west of Scotland, where one of the 'newer granites' enters 

 the Moine schists, and J. J. Sederholm,-" in dealing with 'Adergneis ' in 

 Finland, extended Lawson's views in a new field of regional metamorphism. 



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, while referring back to Hutton as the pioneer, 

 shows how in the vein-gneiss stage the unmelted sediments exhibit plasticity and 

 become intensely contorted. The softening, in fact, induces flow. There is 

 here no crushing or mylonitisation, but rather a viscid running of constituents, 

 some on the verge of fusion, some, I venture to think, actually fused. Such 

 rapidly repeated and intricate folding in metamorphosed sediments has been 

 descrilDed as ' shearing ' by some authors. Neither in the field nor in thin 

 sections -under the microscope can such a position be sustained. Shearing or 

 attempted shearing may subsequently produce what has been called 'strain-slip 

 cleavage ' in the folds ; but the folding has an earlier origin, and is very often 

 associated with thermal changes. It is most intense when lit par lit injection 

 has set in, and when the whole composite mass has become weak and plastic. 

 The presence of confined water in aiding this plasticity must on no account be 

 overlooked. 



It may be well to illustrate this contention by one or two concrete instances 

 from districts not remote from us at the present time. The noble cliffs of 

 ]\Iinaun in Achill Island have been worn by the Atlantic from a mass of evenly 

 bedded quartzites of Dalriadian age. These are invaded by veins of a very 

 coarse red granite, the main mass of which lies below the present sea-level.^' 

 'The edges of the strata appear fairly horizontal on the cliff-face ; but contor- 

 tion sets in towards the base, and the hard resisting rock has here *' ' under- 

 gone intense crumpling and overfolding, such as one meets with on a large 

 scale in mountain ranges, and this contorted flow seems entirely due to the 

 yielding that has taken place in the region of heating.' The veins have 

 broken in sinuous forms across the folds, just as they do in the intensely con- 

 torted vein-gneisses of Finland. They here represent a late episode, occurring 



" Op. cit. on Eainy Lake, p. 131. 



" Compare P. Termier, ' Schistes cristallins des Alpes occidentales,' C. B., 

 Conqres qiol. internat., 1903, p. 585. 



" 'Note on a Granite Junction in the Eoss of Mull,' Oeol. Mag. 1892, 

 p. 447. Compare T. 0. Bosworth on same area. Quart. Journ. Geol. Soc. 

 London, vol. Ixvi. (1910), p. 376. 



' Ueber eine archaische Sedimentformation im siidwestlichen-Finland,' 

 Bull. Comm. qiol. Finlande, No. 6 (1899), p. 133; and 'Ueber ptygmatische 

 Faltungen,' Newes Jalirh. fiir Min., Beilage Band 36 (1913), p. 491. 



^' Proc. Geol. Assoc, vol. xxiv. (1913), Plate 17. 



G. A. J. Cole, ' Illustrations of Composite Gneisses and Amphibolites in 

 N.W. Ireland,' C. E., Congres geol. internat., Canada (1913), p. 313. 



