60 DR. H. B. GUPPY. 
fathom line lies usually between 25 and 380 miles off the 
coast ; and beyond this, there seems to be a gradual descent to 
deeper water, but the plateau is not sharply defined, and the 
submarine contour is very different from that of the ledge, 
bare of reefs, south of Sandy Cape. 
To what conclusion do these facts tend? If We believe 
that the sharply-defined submarine ledge, characterising the 
southern half of the east coast of Australia, also exists in the 
northern half, but covered and concealed by the Great Barrier- 
reef, then the differences between the submarine profiles of 
the part north and the part south of Sandy Cape are mainly 
to be attributed to the existence of the Great Barrier-reef. 
Unless we can explain the reason why this ledge should end 
where the barrier-reef begins, the presumption arises that 
this submarine ledge, destitute of coral reefs for a distance of 
some 900 miles between Sandy Cape and Cape.Howe, is 
extended along the remainder of the now coral-girt Australian 
coast. Here, then, we seem to be within a measurable distance 
of ascertaining the thickness and bulk of the Great Australian 
Barrier-reef. We assume the existence of an underlying 
ledge or plateau. We now require the nautical surveyor to 
supply us with a few lines of close soundings to the deep 
water in order to enable us to determine the angle of the 
submarine slope. After these data have been obtained, it 
will be possible to model the outline of the coast before the 
existence of the barrier-reef. I should here remark that the 
manner in which the Great Australian Barrier-reef has 
altered the submarine profile of the coast is well shown in 
Mr. Murray’s coloured chart of the depths of the ocean, 
which accompanies his paper in the Scottish Geographical 
Magazine for last January. 
I have now gone far enough to establish the probability, 
judging from the instance of the Australian Barrier-reef, 
that reefs of this class are in reality, and not in appearance, 
situated on the border of a submarine plateau or ledge. Such 
a position, according to the explanation of barrier-reefs, 
first advanced by Le Conte, and supported by myself, presents 
the most favourable conditions for reef-growth, the corals 
being limited on the outside by the depth, and on the inside 
by the sediment in the water. The influences of food-supply 
and currents act subsequently as auxiliary causes. 
What, then, is the explanation of the submarine ledge ? 
The supposition that it is a continuation of the land slope is 
at once negatived by the fact that the slope of the land in the 
reef-encircled islands of the Pacific is usually 6 degrees or 
7 degrees, sometimes only 3 degrees or 4 degrees, but often 
