70 SECTIONAL ADDRESSES. 



of to-day originate on submarine slopes, and that important submarine 

 landslips precipitated by such earthquakes have been described, for 

 instance, in connection with the Tokyo disaster of 1923. Of late years 

 Kendall has reawakened British students to the possibility of recognising 

 earthquake phenomena in the records of the past. I believe that a story 

 of recurrent earthquakes is written in the submarine landslip-deposits of 

 the Logan Slope. These deposits show the following characteristics : — 



(1) Through a succession of geological ages (Cambrian to Middle 

 Ordovician) they repeatedly occur along a particular tectonic zone. 



(2) They are often interbedded among shales of Caledonian facies, 

 whereas their material consists mainly of limestone^ blocks and isolated 

 shells of American facies. 



(3) Walcott has shown that in many instances the fossils contained in 

 the blocks are identical with the isolated fossils of the matrix ; the deduc- 

 tion is that the blocks are often little older than the containing deposit. 



(4) The internal arrangement of the deposits is tumultuous and un- 

 bedded. 



(5) Some of these boulders are gigantic. I have seen one 60 feet long 

 that has ploughed deep into underlying shale. Other boulders have been 

 described 150 feet long. 



Various authors have attempted to explain these deposits as glacial, 

 but Ruedemann has stated in regard to an example of Trenton date 

 (Middle Ordovician) that ' the action of coast ice may, in the writer's 

 judgment, be excluded here on account of the presence of the Trenton 

 fossils, including corals, in the matrix.' Ruedemann's judgment may be 

 applied on similar grounds to many instances of earlier date. It is abso- 

 lutely certain that most of these tumultuous deposits accumulated 

 spasmodically during the growth, close at hand, of the great Ordovician 

 limestone of Laurentia. Geographical exploration, in keeping with 

 chemical physiology, assures us that important limestones are products 

 of warm seas. It seems incredible that this Ordovician limestone platform, 

 during its life-history, should have been intermittently exposed to the 

 ravages of ice-floes, or have become the temporary site of an actual ice- 

 sheet. 



If now we cast our minds back to the change of facies that Lapworth 

 recogni'^ed in the Southern Uplands of Scotland we find it on the whole 

 of more gradual type than that characteristic of Canada. In the Southern 

 Upland sea mechanical sediment travelled down a tectonic slope, and 

 change of facies depended upon the arrest of coarse material by deep 

 water. In the Canadian sea mechanical sediment reached the foot of a 

 tectonic slope up which it was unable to climb. In both cases we notice 

 subsidence preceding mountain elevation. This has long been a favourite 

 idea with tectonists. It had its beginnings in a publication of Hall's on 

 the Appalachians, dated 1859. Its subsequent development is due more 

 especially to Dana and Haug. 



We must now recross to Europe, there to get in touch with the later of 

 the two great Palaeozoic chains that meet in South Wales. In 1887 this 

 later chain received a double name from Suess, who distinguished along 

 its course a couple of congruent mountain arcs with an inflectional junction 



