194 CARNEGIE INSTITUTION OF WASHINGTON. 



erosion of the very recent lavas between Nuu'uli and Leone has just begun. 

 The history of Pago Pago Bay, the chief harbor of American Samoa and one 

 of the noblest in the Pacific, presents a special problem, the discussion of which 

 is postponed. 



The detritus resulting from the deep erosion of Tutuila was built into an 

 encircling offshore shelf, the edge of which is now in general 2 to 3 miles from 

 the headland cliffs of the island. Near that edge an interrupted barrier coral- 

 reef was formed. Presumably fringing reefs were contemporaneously devel- 

 oped. Then Tutuila sank by a differential movement, so that the crest of 

 the barrier reef is now from 30 fathoms to a few fathoms below sea-level. 

 Accordingly, the shelf inside that reef is covered with water reaching a depth 

 of 60 fathoms, or more than twice that ruHng in typical barrier-reef lagoons of 

 similar breadth. The subsidence also helps to explain the numerous embay- 

 ments due to the drowning of valleys. 



Prolonged special study has confirmed Mayor's conclusions that when the 

 sea stood at its higher level, corals were not growing on the shores of Tutuila, 

 and that reef corals were absent during the time required to cut the modern 

 chffs on all headlands and even in the shallower bays. The cliffs reach 300 

 or more feet in height. After their cutting the sea-level sank about 20 feet 

 New chfflets were cut at the new level, but protection from the surf was soon 

 furnished, at most of the headlands, by the very recent growth of fringing 

 reefs, which at the same level are now growing outwards. They have at- 

 tained, however, widths usually less than 600 feet and are much narrower 

 than most fringing reefs in the Pacific. 



The evidence for the 20-foot shift of sea-level is founded on the fact, already 

 noted by Mayor, that on all sides of Tutuila the headlands are faced by wave- 

 cut benches with summits approximately 8 feet above present high-water 

 level. Since similar benches, now being cut by the waves, are covered by 

 10 to 15 feet of water about tide, it is necessary to place the high-water level, 

 just before the last shift, about 12 feet above the bench crest-level. Cor- 

 roboration oi this view was found in the discovery of large sea-caves with 

 floors at appropriate heights (14 to 15 feet) above high tide. 



Wave-cut benches at exactly similar levels were found on Aunuu Island 

 and on Tau Island, which is 60 miles from Tutuila. Recent downward shifts 

 of sea-level of the same order of magnitude have been reported from many 

 other Pacific islands, as well as from the east coast of North America and else- 

 where. A compilation of these facts suggests that the last downward shift 

 of sea-level in Samoa is eustatic, world-wide, due to an independent lowering 

 of the ocean surface everywhere. The alternative hypothesis, that the shift 

 was local, because controlled by uplift of the islands, is highly improbable, 

 for vertical movements of the earth's crust are not likely to be uniform at 

 many points, some of which are 60 to 75 miles apart— much less at points 

 hundreds of miles apart, as in the Gulf of St. Lawrence and along the eastern 

 coast of the United States. 



It may be added that there is no conclusive evidence of uplift anywhere in 

 Tutuila. Friedlaender reported elevated limestone 20 meters above sea at 

 two localities in the eastern part of the island. Careful search showed no 

 trace of limestone at these localities, though at one place coral-heads have been 

 carried up by the natives from the reefs to and beyond the height mentioned. 



New light was thrown on one cause for the subsidence of volcanic islands; 

 repeated observations showed that the scoriaceous and "aa" phases of the 

 Samoan lava-flows have been crushed and compacted by the weight of younger 

 lava-flows and pyroclastics. As long as this process continues in depth, the 

 surface of the volcanic pile must tend to sink. 



