^:^6 JOSI'iril HARRELL 



distance of about 150 km. from crest to trough and a wavc-leiigth 

 of 400 500 km., these folds and many others exhibiting a hick of 

 symmi'try. The exislence of strong ft>lding pressures and a 

 tendeniy to overthrusting and secondary \ertical faulting seem to 

 be expressed therein. The great I old passing north from New 

 Zealand and showing as the Kermadec and Tonga islands with 

 their fore-deeps gives a distance from crest to trough of 1 20-180 km., 

 a wave-length of 400 -500 km. Lower California and the troughs 

 on each side show a wave-length o{ ^^50 km. The region of the 

 Lesser Antilles is tectonically a northward branching of the Andean 

 mountain system and shows, like the l\)lds of the Pacilic, crustal 

 iinclulations with a wave-length of 350 km. We may conclude 

 then that these folds of the ocean lloor have a. marked tendency to a 

 wave-length of 300-500 km., there being commonly one great asym- 

 metric foKl passing out into subordinate marginal folds. The vol- 

 canic chain of the 1 lawaiian Islands shows, however, no relatetl deeps 

 ami has a half waAclength o\ about 200 km. 



lla\ ford and Ht>wie have given the New Method anomahes 

 for a few stations in these regions.' A portion of the data has 

 been abstracted and given in Part iV of the present article.'* 

 l'\)ur observations of Uecker's for the Tonga Plateau and Tonga 

 Deep are given. They max not be of high value, since the 

 methinl has been criticized as not possessing accuracy compar- 

 able to observations made by pendulum wyton land. l'\uther- 

 more. the \o\w v>bser\ations, two o\cr the plateau and two oncv 

 the deep, are si>read through a distance of 5,100 km. along the axis 

 of the structure instead of being taken on a transverse section. 

 Nevertheless, as tlu^ reliefs and the correspondii\g anomalies 

 ari^ all of i^rrat magnitutle, the errors become relativeb- small and 

 the mean o\ the observations is therefore of some value. The two 

 New Method anomalies for the Tonga Plateau give a mean of 

 -I-0.202 d)ne, the dcj^lh ol water biang 2,700m. The two New 

 Method anomalies for the Tonga Deep give a mean o\ — 0.17J 

 dyne, the depth o\ water averaging 7,500 n\. If the amplitude 



« The Effect of Toposraphy otui Isosiolic Cotiiprnsaiioii upon the tutciw^ily of GrovHy, 

 191 2, p. 8r. 



' Jour. G(ol., WW oo>.0.,o>- 



