AGASSIZ: THE GKEAT BAKRIER REEF OF AUSTRALIA. 135 



A coral reef, therefore, is a mass of brute matter, living only at its outer surface, 

 and chiefly on its lateral slopes. It is believed that coral animals cannot live 

 at a great depth ; that twenty, or at most thirty fathoms, is their extreme limit 

 of growth. This is apparently proved, or nearly so, with respect to all known 

 species of coral that form reefs, all those found in the hollows or on the shel- 

 tered slopes of reefs, where alone they can be examined. Whether it be uni- 

 versally true, for all polyps depositing large masses of calcareous matter, will 

 perhaps admit of a doubt." 



Juke's account of the impressions produced by a visit to the outer 

 line of reefs is well worth repeating : — 



« Anchored under the lee of a sandbank [Barrier Reef entrance, near Cape 

 Melville], on which I was able to land. This reef was about two miles 

 long, and one mile broad. . . . The sand was wholly calcareous, grains of 

 triturated corals and shells. . . . The outside edge of the reef was of course 

 unapproachable, but the inside I examined as we passed it. The reef sloped 

 gradually at its edge, from a depth of one or two feet to about four fathoms. 

 ... At this depth the white sandy patches at the bottom could be distinctly 

 seen among the large dark masses of living coral. Immediately beyond this 

 the lead sank to ten fathoms. . . . The sand banks are invariably on the lee 

 side of the reef they are upon, which shows the nature of their origin." 1 



Jukes paid a visit to the wreck of the " Martha Ridgway," which lay 

 on the outer Barrier Reef off Raine Islet. Of this visit he says : — 



" The long ocean swell being suddenly impeded by this barrier [the outer 

 line of the Barrier Reef] lifted itself in one great continuous ridge of deep blue 

 water, which, curling over, fell on the edge of the reef in an unbroken cataract of 

 dazzling wbite foam. Each line of breaker was often one or two miles in length, 

 with not a perceptible gap in its continuity. After recovering from this leap, 

 and spreading for some distance in a broad sheet of foam, the wave gradually 

 swelled again into another furious breaker of almost equal height and extent 

 with the first, and then into a third, which, although much less considerable, yet 

 thundered against the bows of the wreck with a strength that often made her ev- 

 ery timber quiver. Even then the force of the swell was not wholly expended, 

 two or three heavy hues of ripple continually traversing the reef, and breaking 

 here and there against the knobs and blocks of coral that rose higher than usual. 

 There was a simple grandeur and display of power and beauty in this scene, as 



fathoms water on it can be discerned at a mile distance from a ship's masthead, in con- 

 sequence of its greenish hue contrasting with the blue deep water. In seven fathoms 

 water the bottom can still be discerned on looking over the side of a boat, especially 

 if it have patches of light-colored sand. But, as I have already stated elsewhere, 

 with a water glass it is possible to obtain excellent views of the bottom at somewhat 

 greater depths. 



1 Voyage of the " Fly," Vol. I. p. 103. 



