FORMATION OF CORAL REEFS AND ISLANDS. 225 



be readily distinguished at a depth of at least twenty metres. 

 The hollows between such ridges or zones are occupied by the 

 heavier substances of the bottom. Similar ripple-marks were 

 distinguished at a depth of one hundred and eighty-eight 

 metres, to the northwest of the St. Paul's Roads. 



In an article on the Force of Waves, by Thomas Steven- 

 son, of Edinburgh, published in the Transactions of the Royal 

 Society of Edinburgh (vol. xvi., 1845), it is stated as a deduc- 

 tion from two hundred and sixty-seven experiments, extend- 

 ing over twenty- three successive months, that the average 

 force for Skerryvore, for five of the summer months, during 

 the years 1843, 1844, was six hundred and eleven pounds per 

 square foot ; and for six of the winter months of the same 

 year, it was two thousand and eighty-six pounds per 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 six thousand and eighty-three pounds w^as regis- 

 tered by Mr. Stevenson's dynamometer (the name of the in- 

 strument used). He mentions several remarkable instances of 

 transported blocks. One of gneiss, containing five hundred 

 and four cubic feet, was carried by the waves five feet from 

 the place where it lay, and there 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 high-water level. On the incoming waves striking the 

 stone, we could see this monstrous mass, of upwards of forty 

 tons weight, lean landward, and the back-run would uplift it 

 again with a jerk, leaving it with very little water about it, 

 when the next incoming w r ave made it recline again." 



15 



