AGASSIZ: THE GREAT BARRIER REEF OF AUSTRALIA. 139 
ward, or towards the east, Now suppose the coast cleared of coral reef, and 
raised so much that it emerged from the sea just within the line of the 
present Barrier Reef. Then let the reef commence in the shallow water along 
that shore, and a very slow and gradual depression take place, giving time for 
the polyps to build up so as to keep near the surface of the water, The result 
of this action would be the present Barrier with its steep outer slope, and its 
gradual extension over the sinking rocks that were once dry land within it. 
Portions that were once hills on the dry land would now be islands between 
the Barrier and the main, such as Sir C. Hardy's Island and those about it. 
Islands that once existed in front of the mainland would now be altogether 
submerged, and their places only marked by detached reefs outside the Barrier, 
such as those north and south of Wreck Bay. According to the old rule of 
high land and deep water going together (in other words, the slope of the 
ground below water being only a continuation of that above), we should have 
the Darrier much closer to the present land in its more abrupt and lofty por- 
tions than in those which were lower and less highly inclined. We see accord- 
ingly the reefs approach the present land about Cape Melville, where the land 
is steep and lofty, and recede from it as we go further north in proportion as 
the land becomes flatter and more gentle in its inclination. Deep holes and 
ravines, full perhaps of fresh water, may have existed on the old land, so that 
when the surface of these lakes and hollows first sank to the surface of the 
sea, and admitted its waters, the bottom may have been too deep for the eoral 
animals to live on. This would explain such a phenomenon as the deep nar- 
row channel just north of Sir O. Hardy's Islands, with reefs running along 
each side of it. In short, every modification in the form and structure of the 
reefs is explicable by this hypothesis, and many difficulties solved, which admit 
of no other explanation.” 
He assumes, as we do, that the Australian coast at one time was just 
within the line of the present Barrier Reef; but it seems to me that 
the causes given by Jukes for the formation of the Barrier Reef are 
equally well explained by erosion and denudation. He assumes a 
great thickness for the corals on the outer edge of the Barrier Reef, — 
a thickness to have grown by the synchronism of the subsidence and 
the growth of the corals, — a thickness the extent of which no one can 
even guess at. We assume for the corals a thickness that can be deter- 
mined fairly accurately as only a veneer of at most twenty fathoms 
upon the faces of the denuded platforms of the islands which once formed 
the outer line of the australian continent, the remnants of which are 
still left as monuments of such a connection in the numerous islands 
seattered along the whole length of the Queensland coast, at various 
distances between the mainland and the line of reefs forming the outar 
edge of the Great Barrier Reef. The geological structure of these 
