XXli INTEODUCTION. 



material, and below that must have, judging by analogy, penetrated either 

 an underlying mass of tertiary limestone similar to that of the raised 

 tertiary limestones of Tonga, Fiji, Niue, Nauru, and Paanopa, or have 

 passed through the mass of recent reef rock forming the outer talus of the 

 atoll of Funafuti. Groups like the Ellice, Gilbert, and Marshall Islands 

 give us the means of studying the many modes of formation of tlie land 

 rims in a most satisfactory manner. Nowhere have we been able to follow 

 as clearly the results of the various agencies at work in shaping the end- 

 less variations produced in the islands and islets of the land rims of the 

 different atolls of these groups ; changes due either to slight elevation or 

 to the incessant handling and rehandling of the older material in place, 

 or of the fresh material added from the disintegration of the sea or 

 lagoon faces of the land rim, or of the corals on the outer and inner 

 slopes. It has been most interesting to trace the ever-changing conditions 

 which have produced so many variations in the appearance and structure 

 of the islands, islets, and of the land rims of the different groups. 



It is of course most difficult to form an opinion regarding the original 

 dimensions of an atoll before it spreads laterally after having reached 

 the surface ; its summit naturally becomes wider as it approaches the sur- 

 face and its talus may be spread as a thin sheet or disappear entirely in 

 the deep waters adjoining an atoll. 



The Fiji group consists of volcanic islands of elevated coralliferous lime- 

 stone islands and of islands partly volcanic and partly of limestone. Two 

 of the volcanic islands are of great size and flanked by wide barrier and 

 fringing reefs, as well as by belts of older elevated coralliferous limestone 

 of tertiary age and by stratified beds of the so-called soapstone of Fiji. A 

 number of the volcanic islands are surrounded by encircling reefs enclos- 

 ing lagoons of considerable depths. 



In the Lau group some of the elevated limestone islands attain a heiglit 

 of over 1000 feet ; they have a wide reef platform and are more or less hat- 

 shaped. In others the original limestone mass has disappeared ; nothing 

 is left, as in Argo for instance, to show its existence except a diminutive 

 limestone island on the outer belt of the encircling reef. 



We find all the intermediate stages between a solid mass of corallifer- 



