MUSEUM OF COMPARATIVE ZOOLOGY. 141 



different types of coral reefs can be explained by the action of known 

 forces. The moment corals have begun to grow, there is nothing to 

 show that they are not at once subjected practically, though in a less 

 degree, to the same conditions as exist at the surface, since a more or 

 less extensive talus is formed in the sea just as at the sea level. The 

 apparently simple method of continuing the slope of the land into the 

 sea, and thus figuring out the depth of the reef, seems to me a most 

 fallacious one. Let us look at the various sections which are known 

 on our northern coast off the Bahamas, off the coast of Florida, off the 

 Windward Islands, and off the coast of Georgia. These are all of differ- 

 ent types, and in a region of coral growth would lead to very different 

 conclusions. The Florida section, which has been given with consider- 

 able detail, 1 is perhaps one of the most interesting. The great mass of 

 observations since the promulgation of Darwin's theory is on the side 

 of the more recent explanation of the formation of coral reefs, while the 

 older theory rests upon an hypothesis of which it is under most circum- 

 stances extremely difficult to obtain any proof whatever. 



Doctor Guppy, 2 who spent considerable time in studying the Solomon 

 Islands, and more particularly the geology and the formation of the 

 calcareous limestones and reefs of the group, altogether dissents from 

 Darwin's explanation of the formation of such a reef as he observed. 



Guppy, in his memoir on the calcareous formations of the Solomon 

 Group, has plainly shown that in that group of islands upraised reef 

 masses, whether atoll, barrier reef, or fringing reef, have been formed 

 in a region of elevation, and such upraised reefs are of moderate thick- 

 ness, their vertical measurement not exceeding the limit of the depth of 

 the coral reef zone, — one hundred and fifty to two hundred feet at the 

 very outside. While this is undoubtedly the case where the reef masses 

 rest upon a foundation of volcanic or older submerged rocks, yet the 

 presence of coral reefs upon foundations of modem limestone, as in the 

 West Indies, made up of fragments of the calcareous remains of all kinds 

 of invertebrates, among which may be deep-sea corals, makes it difficult 

 to fix very accurately the limit of demarcation between the reef lime- 

 stone proper and other recent limestones when both have been modified 

 and changed in elevated areas into the hard ringing compact limestones 

 so characteristic of all areas of elevation. At the Solomon Islands, the 



1 A. Agassiz, The Tortugas and Florida Reefs, Mem. Am. Acad., 1883. 



2 Guppy, H. B., Suggestions as to the Mode of Formation of Barrier Reefs in 

 Bougainville Straits, Solomon Group, Proc. Lin. Soc. of New South Wales, IX., 

 1884, p. 949. 



