June 24, 1921] 



SCIENCE 



565 



reefs. Whether modern observers will adopt 

 his terminology in this respect or not is aside 

 from the point here at issue. The fringing 

 reefs which Darwin regarded as the early stage 

 of barrier reefs may have been either on-shore 

 reefs or ofi-shore reefs; but if ofi-shore, the 

 belt of water between them and the land must 

 have been shallow. This is made perfectly 

 clear by his explicit statement: 



Fringing reefs on. steep coasts are frequently 

 not more than from 50 to 100 yards in width; 

 they have a nearly smooth, hard, surface, scarcely 

 uncovered at low-water, and without any interior 

 shoal channel, like that within those fringing reefs 

 which lie at a greater distance from the land 

 (55, 56). 



These citations leave no doubt that, when 

 the now-submerged barrier reef of Tutuila 

 was fii'st formed at a distance of a mile or two 

 from the cliffed inner border of its shallow sup- 

 porting platform, it would have been classed 

 by Darwin as a fringing reef, because the 

 " enclosed water channel " was then of small 

 depth. But when subsidence continued and 

 permitted reef upgrowth to such a height that 

 the enclosed water channel or lagoon was in- 

 creased in depth, then Darwin would have 

 called it a barrier reef, as modern observers 

 are united in doing. 



The principle here involved is, however, of 

 no great importance, because it was not Dar- 

 win but his successors who have emphasized 

 the supposedly necessary sequence of fringing 

 reef, barrier reef, and atoll, as a consequence 

 of the theory of subsidence. Darwin's own 

 discussion recognized this succession as char- 

 acterizing the typical example of a subsiding 

 island, like Bolabola; but he explicitly recog- 

 nized other sequences in less typical cases. If 

 the original reef foundation had been a bank 

 close to sea level, the initial fringe would have 

 become an atoll, as subsidence progressed, 

 " without passing . . . through the intermedi- 

 ate form of a barrier reef," as a quotation 

 (101) already made shows clearly enough. Or 

 if a reef grew up from the rim of a still- 

 standing submarine crater of proper depth, a 

 possibility which Darwin explicitly recognized 

 (89), an atoll would be formed without the 



preliminary formation of a fringing or a 

 barrier reef. The point of all this is that 

 Darwin conceived many conditions imder 

 which coral reefs might be established and 

 transformed, and did not restrict himself to 

 a special form of reef-foundation or a fixed 

 sequence of reef development, even though he 

 understood that the most probable explanation 

 of the majority of barrier reefs is by the more 

 or less intermittent upgrowth of fringing reefs 

 from subsiding foundations, and that the best 

 explanation of most atolls is by similar up- 

 growth from subsiding barrier reefs. 



The object of this article is to point out that 

 the full meaning of Darwin's broad discussion 

 can not be condensed into a rigidly conceived 

 theory, beginning with an island of a certain 

 shape and proceeding through a perfectly defi- 

 nite sequence of transforming reefs. His 

 treatment of the problem was far broader than 

 that, as the citations given above must suggest, 

 and as various other citations would confirm. 

 Far from being inconsistent with his broadly 

 conceived theoretical discussion, the reefs of 

 Tutuila as described by Mayor, Chamberlin 

 and others are remarkably close exemplifica- 

 tions of some of its most significant special 



W. M. Davis 



Haevaed Univeesitt, CAMBEmGE, Mass., 

 May, 1921 



A NOVEL MAGNETO-OPTICAL EFFECT 



Early in April, 1921, while my son, Malcolm 

 Thomson, was operating, in a building of the 

 River Works plant of the General Electric 

 Co., a resistance welder for closing the seams 

 of steel Langmuir mercury vacuum pumps, in 

 which work the current is applied and cut ofi 

 at about one half second intervals, there was 

 noticed by one of the working force, Mr. 

 Davis, who happened to be favorably located, a 

 peculiar intermittent illumination of the space 

 near the welder as the current went on and off. 

 My son at once placed himself in a similar po- 

 sition and saw the novel effect, and noted a 

 number of conditions accompanying it, per- 

 haps the most important being that a single 

 turn loop from the welding transformer to 



