GEOLOGY. 241 



phytes constitutes in science the order of Actinoidea, a name given on 

 account of their radiated or star-like shape. All the varieties are 

 found in the greatest luxuriance in the waters of the Feejee group. 

 None were found growing deeper than 20 fathoms. 



Mr. Dana mentions the various theories which have been proposed 

 to account for the form and origin of these coral islands, but that 

 adopted by him is the one advocated by Dr. Darwin. He supposes 

 the peculiar form of the reefs to arise from their being built around 

 heights of land, which, by some change in the economy of nature, has 

 gradually subsided. If we suppose a large island or continent to sink, 

 so that the mountains should only remain above the surface, they 

 will, it is clear, form islands, around which the coral zoophyte, which 

 never vegetates below 20 fathoms, will begin its fringe or reef. Let 

 the change of level go on as before, the land, year by year, becoming 

 more and more submerged, and the reefs will draw inwards around 

 the high peaks, and finally, as they go under, will still remain a ring 

 above them with a lagoon ; for the animal could not work in the mid- 

 dle until the whole was submerged, and even then prefers the open 

 ocean. This process going on for ages, so satisfactorily explains all 

 the peculiarities of form found in the coral islands, that it is not easy 

 to avoid the belief that this is the true supposition. 



Leaving the coral islands, Mr. Dana next speaks of the Hawaiian 

 group, and his account of them is no less interesting than that of the 

 coral islands. The eight islands of the Hawaiian group lie between 19 

 and 22^ north latitude. They are Hawaii, Maui, Kahoolawe, Lauai, 

 Molokai, Oahu, Kouai, and Nichau. They extend in a curved line 400 

 miles, and, including the small islets of Necker and Bird, and some 

 coral reefs which properly belong to them, nearly 2,000 miles. They 

 would appear to be the summits of two parallel ranges of mountains 

 or volcanic centres, of which the volcanoes Mount Loa and Mount 

 Kea, in Hawaii, are the southeastern extremities. These mountains 

 are of nearly equal heights ; Loa, according to the measurements of 

 the expedition, is 13,760 feet above half-tide; Kea, 13,950. Mount 

 Hualalai, on the same island, is about 10,000 feet. On Maui, next to 

 the west, Haleakala 10,217 feet, and Eeka 6,130 feet. Oahu has two 

 ranges 4,000 feet, and the summit of Kaui is 8,000 feet. 



Hawaii is nearly triangular in form, its three sides fronting west 85 

 miles, southwest 65, and northwest 75 miles ; its area is 3,800 square 

 miles. Its whole surface is made up of the cones of its three moun- 

 tains, whose slopes are so gentle that the eye scarcely perceives their 

 altitude. In a tour around the island Mr. Dana found the surface to 

 consist chiefly of broad fields of various leaves covered sometimes with 

 a thin soil and dwarf forests, and with occasionally intervening patches 

 under cultivation where the natives raise taro and yams. Sometimes 

 the lava would be smooth and solid ; at others, in fields of scoria and 

 lava in immense masses heaped together in the wildest confusion. 

 These are called clinker fields, and are caused by the lava in its flow 

 melting some obstruction, and cooling and hardening on the surface; 

 then bursting out afresh and rending the crust into fragments, like 



21 



