380 



PHYSICAL GEOGRAPHY. 



revolutions, proceeding from some expansive power which has lifted up, broken, and over- 

 turned their masses in a thousand ways. The lowest and most level parts of the earth, 

 says Cuvier, when penetrated to a very great depth, exhibit nothing but horizontal strata 

 composed of various substances, and containing, almost all of them, innumerable marine 

 productions. Similar strata, with the same kind of productions, compose the hills even to 

 a great height. Sometimes the shells are so numerous as to constitute the entire body of 

 the stratum. They are almost every where in such a perfect state of preservation, that 

 even the smallest of them retain their most delicate parts, their sharpest ridges, and their 

 finest and tenderest processes. They are found in elevations far above the level of every 

 part of the ocean, and in places to which the sea could not be conveyed by any existing 

 cause. The summits of the Pyrenees and of the Andes, at the height of 13,000 or 14,000 

 feet above the level of the sea, present them to our notice. These facts bear witness to 

 the great and wonderful changes which have marked the ancient history of the earth ; for 

 it is obvious that the present continents once occupied a submarine position, from which 

 they have been uplifted a change analogous to that involved in the formation of new 

 islands by a process of elevation, and probably brought about by the same agency, though 

 acting with an incomparably greater energy. 



While the more terrible and destructive instruments of nature occasionally interpose 

 with a disturbing effect in oceanic regions, there is another class of interesting and exten- 

 sive changes in constant process, wrought by the peaceful labours of organic life. In 

 inappreciable numbers, the coral insects minute and apparently insignificant agents 

 swarm in the bosom of the deep, the architects of the production called coral. This was 

 generally deemed a vegetable substance until the year 1720, when M. de Peyronnel of 

 Marseilles commenced, and continued for thirty years, a series of observations, by which 

 he ascertained the coral to be the production of a living animal of the polypi tribe. The 

 general name of zoophytes, or plant-animals, has since been applied to these marine 

 insects, though sometimes called lithophytes, or stone-plants. Various species are in- 

 cluded in the genera, but the most abundant is the muricated madrepore, madrepora 

 muricata of Linnaeus. They occur most frequently in the tropical seas, and decrease in 

 number and variety as we approach the poles. 



One of the coral islands, visited by the American expedition, is described by Captain 

 Wilkes as showing three distinct stages of shelving coast : the one submerged, narrow, and 

 dipping rapidly ; the other broad, level, and covered at high water, but quite bare at low 

 water. On the upper ridge was the usual accumulation of coral debris and sand on which ve- 



Blocks of Coral. 



getation takes place. The annexed cut represents the form of several of the coral blocks seen 

 by the expedition on the Paumotu group of islands, some of them being ten feet by twenty. 

 The most remarkable island of coral formation observed by this expedition, was Metia 

 or Aurora Island, in lat. 15 49' S., long. 148 13' W. It was totally different in appear- 

 ance from any they had yet met with, being a coral island, uplifted, and exposing its 



