1 66 HIDDEN BEAUTIES OF NATURE 



It is an advantage to them if their reef be in a sloping 

 position influenced by tides and currents. The author 

 of The Voyage of the Blake says : ' It is not an un- 

 common thing, after a blow, to come upon this water, 

 discoloured by the fine calcareous silt, to a distance of 

 six to ten miles from the outer reef.' He is here 

 speaking of the coral formations of the Tortugas. 

 We have no definite information as to the rate at 

 which corals grow ; but naturalists of the future will 

 be able to make fairly approximate calculations in 

 this direction, owing to the care which Mr. Saville 

 Kent has taken in his measurements of characteristic 

 blocks of coral, the locality of which he has carefully 

 mapped out. It is the opinion of Prof. A. Agassiz, 

 in describing the Florida reef, that it would take from 

 a thousand to twelve hundred years for corals of this 

 locality to rise from the seven-fathom line to the sur- 

 face. ' This,' he says, ' would give no clue whatever 

 to the actual age of the reef, because it is difficult to 

 determine how far the width of any coral reef is due 

 to the growth of coral. But supposing the reef to 

 have an average width of half a mile, and its lateral 

 growth to be say four or five times more rapid than 

 its vertical increase, we should get at least 20,000 

 years as the age of the outer reef.' 



A question that at once presents itself is : How 

 then about the time when the great solid rocks were 

 formed that are entirely composed of masses of corals 

 of still more remote ages ? We become bewildered 

 with the thought, we have no standard of our own 



