xviii INTRODUCTION. 



into numerous islands and sounds. Both the northern and southern islands 

 are terraced. The Haapai group to the north of Tongatabu is flanked with 

 lovv' islands on the east face, while in the Noniuka group we find elevated 

 coralliferons limestone islands as well as volcanic or limestone islands where 

 volcanic outbursts have pushed through the limestone mass. 



The bank of which Eua Island is the summit has been elevated to a 

 height of over 1000 feet and is separated from Tongatabu by a channel 

 of a greatest depth of not more than 108 fathoms ; a part of the sub- 

 marine plateau to the south of Tongatabu extends as far as Pylstaart. 

 The banks forming the greater Tonga Plateau are separated by water of 

 a considerable depth, from 200 to 300 fathoms. 



A line of volcanic islands runs parallel with the Tonga Plateau. Two of 

 its summits, Falcon Island and Metis Shoal, are of great interest, as they 

 both perhaps indicate the depths to which the action of the sea may extend 

 in shaping platforms of submarine erosion.' Falcon Island at one time 

 attained a height of 250 feet ; it now forms a breaking shoal ; Metis Shoal, 

 with a depth of fifteen fathoms, was once an island, reputed at fir.st, in 

 1875, to be twenty-nine feet high, subsequently raised to 150 feet. 



Admiral Wharton^ has suggested that the depth to which the action of 

 the waves reaches may be indicated by the change of slope generally taking 

 place off shore, below a depth of eighty to one hundred fathoms, and further 

 that the existence of banks in the open sea at a depth of thirty to forty 

 fathoms may be the limit of depth to which oceanic waves cut down a mass 

 upon which they act.^ He also suggests that we may have in the future 

 an opportunity of testing this by sounding the banks now forming on 

 the former site of Falcon and of Metis Islands in Tonga since their 

 disappearance. 



That such extensive submarine denunation takes place seems to me 



1 Wharton, Sir J. W., "Nature," VI., p. Gil. 



^ Address to the Geographical Section of the British Association, Oxford, 1896 ; " Nature," Feb- 

 ruary 25, 1897. 



^ It is known that the effect of great oceanic currents like tlie Gulf Stream is felt at considerable 

 depth. At the straits of Bernini in 439 fathoms the bottom is swept clean, as it also is to the north of 

 Jupiter Inlet along the east coast of the United States as far north as Cape Hatteras, and along this 

 stretch the Gulf Stream carries a certain amount of silt ; this must act like a hydraulic flume and 

 wear the bottom very perceptibly. This is seeu in the sections across the Gulf Stream given in 

 " Three Cruises of the Blake," Vol. I. p. 258. 



