Dilation of the Great Arteries Distal to Partially Occluding Bands

than in the Pacific ocean.The recent date and the rapid rate of subsidence appear to be of greater importance than its amount in'the case of the Great Chagos bank, where the submergence does not seem great enough to drown the reef-building corals.Here the muddy central area is 40 or 50 fathoms deep; it is bordered by an irregular sandy bank from one to 5 miles or more in breadth and from 15 to 20 fathoms in depth, on the outer margin of which rises a rim about a mile in width, and only 5 or 10 fathoms in depth; singularly enough, there is little living coral on the outer rim, though knobs of growing coral rise from the central depression.The diameters of the whole mass range from 50 to 75 miles: its form suggests that a-prolonged stationary period, during which a broad atoll-reef was developed, was followed by a subsidence of about 10 fathoms, after which a shorter stationary period permitted the upgrowth of a narrower reef; then a rapid and presumably recent subsidence of 5 or more fathoms ensued, since which no effective reef growth has taken place, possibly because, according to Daly's suggestion, the submerged corals were smothered by wave-and current-shifted sediments.Unfortunately no archipelagoes comparable to those of the Australasian region are present in the Indian ocean to give evidence in the case, but it may be noted that a few high islands which occur in association with the Indian ocean banks-chiefly the granitic islands in the area of the great Seychelles bank-have narrow and unconformable fringing reefs on their deeply eroded and well embayed shores; thus they repeat in a small way the more abundant and therefore more compulsory evidence that is provided by the charts of the Philippines.Further details on these topics are given in an article on "Sub- marine Banks and the Coral Reef Problem," now in course of publication in the Journal of Geology, and in an article on the "Subsidence of Reef-encircled Islands," soon to appear in the Bulletin of the Geological Society of America.