320 



C. SKOTTSBERG 



Field observations show that these islands have had histories which present marked 

 differences from those of the continents and their marginal island arcs. Thus, they 

 appear to be composed almost entirely of volcanic material, mainly basic, together with 

 "coral reef" formations, whereas granitoid intrusives and acid lavas are lacking, to- 

 gether with the sediments invariably associated with "continental" areas. And not only 

 is this so, but the volcanic ejectamenta of these inner groups do not appear to con- 

 tain fragments of granitoids and sediments such as might be expected from volcanos 

 discharging through a foundation of rock formations such as compose the continents 

 (p. 203). 



Another interesting feature is the peculiar topography of that portion of the Pacific 

 floor which separates the great island arcs and lands of "continental"' character from 

 the more central groups. Thus, on the American side of the Pacific, the "continental" 

 lands are separated from the groups — such as Hawaii, the Marquesas, the Society Is- 

 lands, and luan Fernandez — by a series of deep discontinuous ocean trenches, prac- 

 tically collinear (p. 203). 



The question is whether these trenches are of quite the same nature and 

 date from the same period as the deeper trenches arranged oceanward from the 

 great western island arcs. It is unfortunate for the advocates of a "continental" 

 origin of the Juan Fernandez flora that these islands are situated on the wrong 

 side of the trench. However, the Galapagos Islands occupy a similar position, 

 and still they have been claimed, on good grounds, once to have been united 

 with Central America. In this connection another quotation from Andrews with 

 regard to the New Hebrides and Tonga is of interest. 



Island groups which are difficult to place exactly in this scheme are the Xew 

 Hebrides, Tonga, and possibly the Pellews and the Ladrones. A profound deep lies 

 between New Caledonia and the Xew Hebrides, and this is suggestive of a noncon- 

 tinental origin of the group. On the other hand, tlie occurrence of mineral deposits 

 such as copper, iron, and nickel, of large kauri, fig, myrtaceous, and other trees, and 

 of animals such as lizards, turtles, ducks, pigeons, and parrots, suggests that they may 

 well have formed, at some earlier time, portions of a continental margin which later 

 became involved in a powerful movement within the marginal Pacific, resulting in the 

 gradual submergence of these outer portions, the present Hebrides, Tonga, and so on, 

 being built upon such sinking area. This certainly is suggested for the New Hebrides 

 and for Tonga, while the Pellew and Ladrone islands also have had complex histories, 

 which would well repay close attention, in their structural, petrological and biological 

 aspects (p. 203). 



An additional point of interest is the association of great ocean deeps with youthful 

 volcanic zones, and inasmuch as the trough and crest of an earth undulation are parts 

 of the same structure, it is a legitimate inference that the great Pacific deeps or trenches 

 are relatively youthful structures (p. 203). 



Andrews summarized his idea of the Pacific basin in a number of points, 

 which, with very slight verbal alterations, form the introduction to his paper on 

 the origin of the Pacific insular floras (7. 613-14): 



1. The continents bordering the Pacific have been larger, at various times, than 

 they are at present. 



2. The great bordering island arcs of the Pacific — such as the Aleutians, Japan, 

 the Philippines, the Netherlands East Indies, New Guinea, Fiji, New Caledonia, and 

 New Zealand — have been connected directly with the continental lands. Certain of these 



