1836.] FORMATION OF ATOLLS. 459 



from the general foundation before any other part, and 

 that this would account for the ring or cup -shaped 

 structure. But we shall immediately see, that in this, as 

 well as in the crater-theory, a most important consideration 

 has been overlooked, namely, on what have the reef- 

 building corals, which cannot live at a great depth, based 

 their massive structures ? 



Numerous soundings were carefully taken by Captain 

 Fitz Roy on the steep outside of Keeling atoll, and it was 

 found that wi'thin ten fathoms, the prepared tallow at the 

 bottom of the lead invariably came up marked with the 

 impression of living corals, but as perfectly clean as if it 

 had been dropped on a carpet of turf; as the depth 

 increased, the impressions became less numerous, but the 

 adhering particles of sand more and more numerous, 

 until at last it was evident that the bottom consisted of a 

 smooth sandy layer : to carry on the analogy of the turf, 

 the blades of grass grew thinner and thinner, till at last 

 the soil was so sterile, that nothing sprang from it. From 

 these observations, confirmed by many others, it may be 

 safely inferred that the utmost depth at which corals can 

 construct reefs is between twenty and thirty fathoms. Now 

 there are enormous areas in the Pacific and Indian Oceans, 

 in which every single island is of coral formation, and is 

 raised only to that height to which the waves can throw up 

 fragments, and the winds pile up sand. Thus the Radack 

 group of atolls is an irregular square, 520 miles long and 

 240 miles broad ; the Low Archipelago is elliptic-formed, 

 840 miles in its longer, and 420 in its shorter axis ; there 

 are other small groups and single low islands between 

 these two archipelagoes, making a linear space of ocean 

 actually more than 4000 miles in length, in which not one 

 single island rises above the specified height. Again, in 

 the Indian Ocean there is a space of ocean 1500 miles in 

 length, including three archipelagoes, in which every 



land is low and of coral formation. From the fact of 

 tie reef-building corals not living at great depths, it is 

 absolutely certain that throughout these vast areas, where- 

 ever there is now an atoll, a foundation must have 

 originally existed within a depth of from twenty to thirty 

 fathoms from the surface. It is improbable in the highest 



'gree, that broad, lofty, isolated, steep-sided banks of 



f diment, arranged in groups and lines iiundreds of leagues 

 in length, could hnve been deposited in the central and 



