408 TRANSACTIONS OF SECTION C. 



amphibole acquires potassium and breaks down into biotitc, and biotite-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 sub-crustal 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. 



To realise the magnitude of the process, however, hand-specimens and museum- 

 specimens will not suffice. Here, as in all branches of geology, travel is the 

 best of teachers, and the finest illustrations in a Geological Survey memoir 

 will not convey the same impression as one wave-snvept island of the Finnish 

 skaga.rd,_a glaciated hillside in Donegal, or a dome of the Laurentian peneplane, 

 from which the forest has been burnt away. 



I am aware that in this statement of the relationf; of the Archaean gneisses to 

 the overlying floor I am neglecting the effects attributed to dynamometa- 

 morphism. A rise of temperature, leading to molecular readjustment, is usually 

 admitted by those who lay stress on the evidence of pressure, and etudy in the 

 field along what may be called regional contacts produces the impression that 

 thermal influence is a very potent factor. This is not the place for a review 

 of the position taken by French observers," who have done so much to enlarge 

 our ideas of contact-alteration and intermingling. It is sufficient to remark that 

 such contact-alteration, acting over wide areas, including as it does the advance 

 of permeating liquids, goes far to account for the ' mineralisation ' of previously 

 normal sediments, while the injection of which there is such atondant 

 evidence will explain the numerous varieties of composite and banded gneiss. 

 J. Lehmann -" restricted the term ' gneiss ' to foliated igneous rocks in which the 

 parallel structure was due to original flow or subsequent crushing, and denied the 

 existence of a passage from mica-schist to gneiss. He left no place in his 

 classification for the composite rocks that furnish so much information as to 

 the methods of sub-crustal fusion. Wlien such an authority as J. P. Iddings '^ 

 hesitates to accept either the assimilation or the stoping theory as explaining 

 the advance of an igneous magma on a large scale in the crust, I feel that I may 

 not be able to carry all my hearers with me in this part of my argument as to 

 the causes of terrestrial collapse. I can only say that I support certain con- 

 clusions, because I can conceive no other reading of the evidence offered in the 

 field. Wlien it is asserted that the earth is not hot enough to allow of the 

 melting of one rock by another, I can only reply that such melting has taken 

 place. The influence of liquids and gases in promoting fusion has been 

 emphasised by Iddings, Judd, and Doelter. Even with this aid, we may not be 

 able to explain the facts, though I think that we have gone a long way towards 

 doing so. In the history of all the sciences, however, observation has run far 

 beyond understanding. 



Fortunately the argument that I hope to develop as to the comparative 

 rapidity and possibly catastrophic character of certain crustal changes depends 

 only in part on views that are still under discussion. 



The invasion of a ' hard and brittle ' °^ crust by an attacking magma was 

 finely described by Lawson in 1888. Lawson pointed out that the Laurentian 



" Michel Levy summarised his views, side by side with essays by other 

 authors, in ' Etudes sur les Schistes cristallins,' C. B., Congres qeol. internat. 

 1888, p. 117. See also P. Termier, ' Les Schistes cristallins des Alpes occi- 

 dentales,' C. R., Congres qeol. internat., 1903, p. 571. Compare numerous later 

 observers, such as P. S. Richarz, ' Umgebung von Aspang,' Verhandl. k. k. 

 ReirJisnn.ttalt, 1911, p. 285. 



'" Op. cit., C.R., Congres giol. internat., 1888, p. 115. 



-'■ Igneous Rocks, vol. i. (1909), p. 282; Tlie Problem of Volcanism (1914), 

 p. 200. 



°- A. C. Lawson, op. cit. p. 140. See also his revision of the area, Gcol. 

 Surr. Canada, Memoir 40 (1913). 



