THE ORIGIN OF CORAL REEFS. 59 
where it is fronted by barrier-reefs, it will be seen that my 
hopes will not altogether be disappointed. 
Let us consider the characters of the submarine profile of 
the east border of Australia from Cape Howe to Torres 
Straits. The northern half of this coast, from the vicinity of 
Sandy Cape northward to Torres Straits, lies within the region 
of coral reefs, limited here roughly by the tropical circle, and 
is fronted by the great Australian barrier-reef. The southern 
half, between Sandy Cape and Cape Howe, lies outside the 
coral-reef region and is bare of reefs; but there exists a 
characteristic submarine ledge stretching southward all along 
the coast from Sandy Cape. This ledge is well defined by 
the 100-fathom line; its width usually varies between 18 and 
25 miles, but it attains double this breadth as we approach 
Sandy Cape. Its slope, represented usually by a fall of from 
20 to 30 feet in a mile, has an angle of only a small fraction 
of a degree. Beyond this ledge, judging from the soundings 
off Wide Bay, Low Bluff, and Cape Byron, the average angle 
of descent to depths between 2,000 and 2,500 fathoms varies 
between 7 degrees and 14 degrees. Here, then, we have an 
undoubted submarine ledge on that portion of the Australian 
coast which lies outside the region of coral reefs, or, in other 
words, outside the tropical circle, the limit in this locality of 
the growth of reef-corals. 
Let us now examine the submarine profile of the region 
fronted by barrier-reefs which extends from near Sandy Cape 
to Cape York. The presumption is, unless by some singular 
coincidence the submarine ledge terminates where the barrier- 
reef begins, that this ledge or plateau originally extended 
along the remainder of the east border of Australia north- 
ward to Torres Straits. We should naturally expect, therefore, 
that there would be a great difference in contour between the 
northern part of the ledge, which has been incrusted for un- 
known ages with the growing mass of the great barrier- 
reef, and the southern part of the ledge, which, being out- 
side the area in which reef-corals live, is destitute of reefs ; 
and such we find to be the case. From Sandy Cape north- 
ward to Flinders Passage, a distance of about 500 miles, the 
100-fathom line extends to distances varying between 60 and 
120 miles from the coast; but we have to proceed between 
30 and 40 miles beyond, and to a depth of about 250 fathoms, 
before we can define the submarine plateau of this part of 
the Australian coast. The contour presents a great contrast 
to that of the submarine ledge south of Sandy Cape, which 
has a breadth of from 18 to 25 miles, and is sharply limited 
by the 100-fathom line. North of Flinders Passage the 100- 
