128 POPULAR ILLUSTRATIONS OF 



years ; although, to allow the most liberal scale in his calculations, 

 he sets it down as one foot in a century. I may here remark, 

 however, that Mr. Darwin states that a channel made with crowbars 

 through the reefs round Keeling Island, according to the authority 

 of Mr. Liesk, only ten years before, was at his visit so blocked up 

 with living Coral that ships could not enter. Mr. Darwin also 

 relates experiments conducted by Dr. Allen, of Forres, on the east 

 coast of Madagascar, who placed twenty Corals on a sand bank in 

 three feet of water (low tide), and in the July following each had 

 nearly reached the surface, and was quite immovable. Mr. Scutch- 

 bury also relates, in the West of England Journal, vol. i. p. 50, 

 that an oyster, only two years old, had attached to it a Coral, one of 

 the Agarica, which weighed 21b. 9oz. There is no doubt, however, 

 that the rate of growth varies with the species, and as Professor 

 Agassiz was well aware of the instance I have cited, as well as the 

 deduction of Mr. Dana from data supplied him by Sir J. G. Dalyell, 

 on the growth of Actinia viz., that the polyp of the Coral 

 Madrepore increased three inches per annum, I presume he was 

 well satisfied with the correctness of his own experiments, and 

 of the deductions drawn from them. These deductions are the 

 following : 



Beyond the mainland of the extremity of the peninsula of Florida 

 are a series of islands known as the "Keys," and of these "Key 

 West" is the most western. Outside these "Keys," and rising just 

 above and resting upon the Florida living reef, are a series of small 

 scattered islands. Now, mark that I have said, "living reef." 

 In fact, the Corals are alive from the surface of the water to the 

 depth of 60ft. Professor Agassiz places the limit of polyp life 

 much less deep than Mr. Darwin, as he says there is no proof that 

 the creature can live below twelve fathoms. 



Calculating, therefore, that the Florida reef is sixty feet deep, and 

 that it has been built up at the rate of twelve inches in a century, 

 Professor Agassiz considers that this reef must have been six thou- 

 sand years in reaching its present height, and that the island out- 

 side the "Keys" must have taken this time to form. 



On examining the "Keys" themselves, Professor Agassiz found 

 that they were nothing more than reefs which had been built up 

 before the outer reef began to form. Soundings gave the same 

 depth of 60ft., and the only difference between the inner reef and 

 the outer was that the crests of the former had been broken by the 

 action of the tides and storms. He then calculated that the age 

 of this old reef was 12,000 years, and he calls this the second item 

 in his calculation. 



Now the "Keys" are separated from the shore by "mud flats," 



