54 A Retrospect of Oceanography 



50 Fahr. (10 C.), and building up a veritable reef, towards 

 which it had already contributed a pedestal some 400 fathoms 

 high. It had also a particular interest for me, because until 

 then there was an objection to Dr Murray's theory of coral 

 islands to which I could never find a sufficient answer. In 

 Murray's theory, as stated by him, the shoals on which 

 the reef-building corals of tropical seas finally settled, and, 

 by their vital activity, increased so as to form an atoll, were 

 formed by the more rapid accumulation of calcareous sediment 

 on the shallower parts of the ocean than on the deeper, the 

 rate of accumulation increasing as the depth diminished. 

 Now, knowing that the effect of our eminence on the bottom of 

 the ocean is not only to restrict and to locally intensify any 

 current that may exist in the region, but also, by interfering 

 with the all-pervading tidal undulation, to convert it, pro 

 tanto, into a tidal current; the effect of either or both of these 

 agencies would be to arrest the growth of a shoal formed of 

 sediment, at a depth so great and in water so cold, that the 

 lodgement on it of tropical reef-builders would be impossible. 

 But with the deep-sea corals stepping in as an intermediary, 

 and taking up the building in depths of 1000 fathoms or more, 

 where the current begins to sweep away as much sediment as 

 falls, thereby stopping its accumulation, and by the same act 

 producing the conditions most favourable for the growth of the 

 deep-sea corals, the difficulty was removed, and the so-amended 

 theory became an accurate expression of the facts. 



While a certain amount of time was spent in the "Dacia" 

 in ascertaining the slopes in the oceanic shoals, which had 

 only a scientific interest, much more time was spent in ascer- 

 taining the slopes leading up to the shores of the islands on 

 which the cable was to be landed. This is the most difficult 

 and delicate part of the submarine-cable engineer's work. 

 The Canary islands are all volcanic, and have been built up 

 from the bottom of the ocean by successive overflows of lava, 

 which takes the form of more or less raised streams having a 

 more or less arched surface, like a glacier. The valleys of the 

 islands are primarily the intervals between these streams, 

 subsequently accentuated by the meteorological decay of the 



