viii INTRODUCTION. 



innumerable islets that have been formed on the coral reefs 

 at the present sea-level, we come first to those small islands 

 and islets less than 100 feet in height, such as the Three 

 Sisters and Stirling Island, which are composed entirely of 

 coral limestone. In the next place there are islands of 

 larger size and greater height, such as Ugi, which are com- 

 posed in bulk of partially consolidated bedded deposits 

 containing numerous foraminifera, and possessing the char- 

 acters of the muds which were found by the "Challenger" 

 Expedition to be at present forming around oceanic volcanic 

 islands in depths probably of from 150 to 500 fathoms. 

 Coral limestones encrust the lower slopes of these islands 

 and do not attain a greater thickness than 150 feet. The 

 next type is to be found in Treasury Island which has a 

 similar structure to that of Ugi, but possesses in its centre 

 an ancient volcanic peak that was once submerged and is 

 now covered over by these recent deposits. Then, there are 

 islands, such as the principal island of the Shortlands, in 

 which the volcanic mass has become an eccentric nucleus, 

 from which line after line of barrier-reef has been advanced 

 based on the soft deposits. These soft deposits contain 

 amongst other organic remains, the shells of pteropods and 

 the tests of foraminifera in great abundance. In such 

 islands I did not find that the coral limestone had a thick- 

 ness of as much as 100 feet. In this island the upraised reefs 

 are based upon hard foraminiferal limestones. Lastly, we 

 have the upraised atoll of Santa Anna which within the 

 small compass of a height of 470 feet displays the several 

 stages of its growth ; first, the originally submerged volcanic 

 peak ; then, the investing soft deposit resembling in character 

 a deep-sea clay and considered to have been formed in con- 

 siderable dei)ths, probably from 1500 to 2u00 fiithoms ; and 

 over all, the ring of coral limestone that cannot far exceed 

 150 feet in thickness. The islands fornjed mainly of the 

 soft foraminiferous deposits have long level summits free 

 from peaks. Judging from their profiles, the islands of 

 Ulaua and Ronongo will be^found to possess the structure of 

 Ugi and Treasury. The western end of Choiscul has a verv 

 significant profile, and I have little doubt from my exami- 

 nation of the lov/er slopes that this extremity of the island 

 is mainly composed of the recent soft deposits. 



I now proceed to refer very shortly to the coral reefs i of 

 these islands. The three principal classes are to be found in 



^ Vide my paper on this subject. (Proc. Roy. Soc, Edin., I88J1-86.) 



