40 Mr Maclaren on Cural Inlands and Heefs, as 



material than drifted coral, resting on the edge of the sub- 

 marine I'eef. The argument holds equally good against the 

 hypothesis of submarine craters ; for so many hundreds of 

 these could not approach within a few fathoms of the surface, 

 without some of them rising above it. 



4. Banks of sediment might (as some suppose) serve for a 

 basis to atolls in shallow seas ; but to assume the existence of 

 hundreds of such banks of moveable matter in the profound 

 depths of the ocean, is absurd ; and it is positively disproved 

 in the case of those atolls whose exterior sides are steeper 

 than the cone of a volcano, descending, as some of them do, at 

 an angle of 40 or 50 degrees. 



The theory adopted, whatever it is, should also explain the 

 existence of barrier reefs, which are analogous to atolls in every 

 point, except that of having solid land within them. How, for 

 instance, on any of the theories proposed, are we to account 

 for the great barrier reef of Australia, with 60 fathoms of 

 water even on its inner side, and descending on its outer side 

 to unfathomable depths at a high angle ? Are we to assume 

 that there is a submarine precipice here 1000 miles in length, 

 on which it rests. 



The only hypothesis, Mr Darwin observes, ichich solves all dif- 

 ficulties, is that ichich assumes that the atolls rest on land which 

 has stihsided, and part of which was once dry. Detached atolls 

 far from others, may stand on submarine rocks which have un- 

 dergone no change of position ; but those found in groups 

 mark the site of land which has subsided. In short, the atolls, 

 according to Mr Darwin's theory, may be regarded as the ves- 

 tiges or foot-prints of land ivhich has disappeared ; and the islands, 

 encircled by barrier reefs, as remnants of land noio partly submer- 

 ged, and perhaps in progress towards final disappearance. 



As the coral animalcules do not live at a greater depth than 

 200 feet, it follows that all reefs, however deep, must have 

 begun in shallow seas ; in other words, they must have been 

 originally of the nature of " Fringing Eeefs." 



Let us suppose an island 350 feet high to exist in the tro- 

 pical seas. The animalculag commence their labours on some 

 spot, and at a distance from the shore, as turbid water is per- 

 nicious to them. But since they cannot exist at more than 



