and Diastrophism in Oceanica. 101 



the Philippines. The Moluccas, however, appear to be geo- 

 logically and structurally a part of iSTew Guinea, and are there- 

 fore remnants of the foundered continent of Australasia (see 

 fig. 1). 



The Oceanides and Foredeeps. 



Oceanides. — Long ago Dana pointed out that there are fifteen 

 chains of islands in the Pacific Ocean, all trending from 

 K30°W. to K65°W. These are a part of the Oceanides of 

 Suess, and Dana states 6 that they "are not independent lines, 

 but siibordinate parts of island chains. There are three gxeat 

 island chains in the [Pacific] Ocean which belong to the north- 

 westerly [trending] system — the Hawaiian [1,500 miles long], 

 the Polynesian [parallel chains 5,500 miles long], and the 

 Australasian [2,000 miles long]." The first two series are 

 chains of oceanic islands built up largely by volcanoes, while the 

 Australasian and the 2,500-miles-long ISTew Zealand chain, with 

 its northeasterly trend, are but the rising northern and eastern 

 boundaries of the otherwise much broken down and foundered 

 continent of Australasia. Here occur andesitic and other rocks 

 characteristic of continents, and also much compressed and 

 metamorphosed formations not seen on the oceanic islands. 



Foredeeps (see fig. 1). — In front of the New Zealand chain 

 lies one of the five greatest deeps of the Pacific, the Tonga- 

 Kermadec-N~ew Zealand foredeep (known as the Aldrich deep), 

 whose deeper parts are between 26,000 and 31,800 feet. There 

 are no known foredeeps along the northern outer boundary of 

 Australasia, but just within the outer chains there have been 

 found three deeps ranging between 19,500 and 29,700 feet — 

 the two Solomon Islands deeps (the northern one is known as 

 the Planet deep), and the unnamed deep between the ISTew 

 Hebrides and iSTew Caledonia. To the east of the Philippines 

 is another, more extensive foredeep attaining to 27,800 feet 

 (Swire deep) ; the Pourtales deep lies 500 miles to the east 

 and the Challenger deep 500 miles still farther east along the 

 eastern side of the Guam-Ladrone or Marianne Islands, with a 

 greatest depth of 31,315 feet. To the east of the Asiatic fore- 

 deeps lies the Pacific Ocean proper, whose average depth varies 

 between 15,000 and 18,000 feet, and on its floor rise the more 

 or less submerged Oceanides. 



6 J. D. Dana, Manual of Geology, 4th ed., 1895, p. 37. 



