a hammer, and inclined at a gentle angle, corresponding with that of the surface of the beach.  The hard parts of the many animals which live upon the reef become imbedded in this coral limestone, so that a block may be full of shells of bivalves and univalves, or of sea urchins; and even sometimes encloses the eggs of turtles in a state of petrification.  The active and vigorous growth of the reef goes on only at the seaward margins, where the polypes are exposed to the wash of the surf, and are thereby provided with an abundant supply of air and of food.  The interior portion of the reef may be regarded as almost wholly an accumulation of dead skeletons.  Where a river comes down from the land there is a break in the reef, for the reasons which have been already mentioned.

The origin and mode of formation of a fringing reef, such as that just described, are plain enough.  The embryos of the coral polypes have fixed themselves upon the submerged shore of the island, as far out as they could live, namely, to a depth of twenty or twenty-five fathoms.  One generation has succeeded another, building itself up upon the dead skeletons of its predecessor.  The mass has been consolidated by the infiltration of coral mud, and hardened by partial solution and redeposition, until a great rampart of coral rock one hundred or one hundred and fifty feet high on its seaward face has been formed all round the island, with only such gaps as result from the outflow of rivers, in the place of sally-ports.

The structure of the rocky accumulation in the encircling reefs and in the atolls is essentially the same as in the fringing reef.  But, in addition to the differences of depth inside and out, they present some other peculiarities.  These reefs, and especially the atolls, are usually interrupted at one part of their circumference, and this part is always situated on the leeward side of the reef, or that which is the more sheltered side.  Now, as all these reefs are situated within the region in which the tradewinds prevail, it follows that, on the north side of the equator, where the trade-wind is a northeasterly wind, the opening of the reef is on the southwest side:  while in the southern hemisphere, where the trade-winds blow from the southeast, the opening lies to the northwest.  The curious practical result follows from this structure, that the lagoons to these reefs really form admirable harbours, if a ship can only get inside them.  But the main difference between the encircling reefs and the atolls, on the one hand, and the fringing reefs on the other, lies in the fact of the much greater depth of water on the seaward faces of the former.  As a consequence of this fact, the whole of this face is not, as it is in the case of the fringing reef, covered with living coral polypes.  For, as we have seen, these polypes cannot live at a greater depth than about twenty-five fathoms; and actual observation has shown that while, down to this depth, the sounding-lead