224 J. D. Dana, Esq., on Coral Reefs and Islands. 



to be found in various treatises on Geology, of still more sur- 

 prising effects.* We must, therefore, allow that some effect 

 will be produced upon the coral groves. There will be trees 

 prostrated by gales, as on land, fragments scattered, and 

 fragmentary and sand accumulations commenced. Besides, 

 masses of the heavier corals will be uptorn, and carried along 

 over the coral plantation, which will destroy and grind down 

 everything in their way. So many are the accidents of this 

 kind to which zoophytes appear to be exposed, that we might 

 believe they would often be exterminated, were they not 



* Lyell, vol. ii., p. 38-40. Speaking of the force of the waves on coasts, 

 Lyell mentions the transportation of a block of stone, ninety feet from its bed, 

 which was eight feet two inches, by seven feet, and five feet one inch in its 

 dimensions, and another nine feet two inches, by six-and-a-half feet, by four 

 feet, having been " hurried up an acclivity to a distance of 150 feet." 



In an article on the subject, by Thomas Stevenson, civil engineer, of Edin- 

 burgh, published in the Transactions of the Royal Society of Edinburgh (vol. 

 xvi., 1845), it is stated, as a deduction from 267 experiments, extending over 

 twenty-three successive months, that the average force for Skerryvore, for five 

 of the summer months, during the years 1843-1844, was 611 lb. per square foot; 

 and for six of the winter months of the same year, it was 2086 lb, j)er square foot, 

 or three times as great as during the summer months. During a westerly gale, 

 at the same place, in March 1845, a pressure of 6083 lb. was registered by Mr 

 Stevenson's dynamometer (the name of the instrument used). He mentions 

 several remarkable instances of transported blocks. One of gneiss, containing 

 504 cubic feet, was carried by the waves five feet from the place where it lay, 

 and then became wedged so as no longer to be moved. Of the manner in which 

 it was moved, Mr Reid (as cited by Mr Stevenson) says : — " The sea, when I 

 saw it striking the stone, would wholly immerse or bury it out of sight, and 

 the run extended up to the grass line above it, making a perpendicular rise of 

 from thirty-nine to forty feet above the high-water level. On the incoming 

 waves striking the stone, we could see this monstrous mass, of upwards of forty 

 tons weight, lean landwards, and the back-run would uplift it again with a 

 jerk, leaving it with very little water about it, when the next incoming wave 

 made it recline again," 



Mr Stevenson states also, that the Bell Rock Lighthouse, in the German 

 Ocean, though 112 feet in height, is literally buried in foam and spray to the 

 very top, during ground swells, when there is no wind. On the 20th of No- 

 vember 1827, the spray rose to the height of 117 feet above the foundations or 

 low-water mark ; and, deducting eleven feet for the tide that day, it leaves 106 

 feet, which is equivalent to a pressure oi nearly three tons per square foot. 



With such facts, any incredulity respecting the power of wavea should be 

 laid aside. Moreover, it may be remarked, that the Pacific is a much wider 

 Ocean than tlie Atlantic, M'ith far heavier waves in its ordinary state. 



