XXX INTRODUCTION. 



There are as yet comparatively few soundings off the sea faces of atolls 

 and reefs to give us more than very general ideas regarding the slope of 

 the upper parts of a cor.al reef. A number of soundings are known taken 

 a distance of half a mile or a mile from the edge of the reef, but within 

 that distance comparatively few data exist. We must except the soundings 

 of the " Challenger " on the outer slope of the barrier reef off Papiete 

 and the soundings off Funafuti by the " Penguin ; " they are more numer- 

 ous than those taken off any other atoll and show great diversity in the 

 slope on the different faces of the atoll. 



All the soundings known indicate that coral reefs rise independently 

 upon summits formed by tertiary limestones or volcanic rocks, summits 

 which have been formed either by elevation or submarine denudation 

 or upon sunmiits of accretion forming submarine banks. 



The nature of the underlying base is naturally an important factor in 

 determining the pitch of the sea slope ; we may assume that the steep 

 slopes of the upper part of reefs is due to the sloughing off of the lime- 

 stone cliffs down to thirty or forty fathoms, much as they are sloughed off 

 from the faces of elevated islands above high-water mark. If composed 

 of volcanic ash or harder volcanic material the slope in one case will be 

 very slight, spreading rapidly laterally under the influence of the waves ; 

 in the other the slope of volcanic material will not differ materially 

 under water from that above high-water mark. When the slope is a talus 

 of reef material it may lie at a steep angle or may follow closely the 

 slope of the underlying base below the depth at which corals or Nulli- 

 pores grow. 



In the Atlantic the slopes off the Florida barrier reef have been carefully 

 sounded; they are not steep, but pass very gradually into deep water. The 

 same is true of the slopes off the Bahamas sounded by the " Albatross " 

 under Captain Tanner, and of those off the Bermudas as determined by the 

 "Challenger." These all nre in marked contrast to the steep sea faces off' 

 the Great Barrier Reef of Australia, off Tahiti, and Fiji. The slope of 

 Masamarhu Island^ in the Red Sea has been carefully surveyed; it is quite 

 abrupt to a depth of about eighty fathoms. Two excellent sections have 



1 "Nature," 1887, p. 413. 



