nut trees, can be seen separating the dark heaving waters of the ocean from the light green expanse of the lagoon, the still waters

of which, within the reef, usually bathie a fringe of low alluvial soil, upon which the varied and beautiful productions of the tropical regions flourish at the foot of the abrupt and wild central peaks. In the sketch given above, the barrier-reef may be seen in the distance skirting around the island. These reefs are of all sizes from three to forty miles in diameter; and the one which encircles both ends and fronts one side of New Caledonia is upwards of 400 miles long. Externally the reef rises like an atoll with abruptness out of the profound depth of the ocean, but internally it either slopes gradually into the channel, or terminates in a perpendicular wall 200 or 300 feet in height.

There is one remarkable feature connected with the circular reefs, and that is, a deep and narrow passage almost invariably opening from the sea into the lagoon, and kept open by the efflux of the sea at low tides, and it has long been remarked in the case of the barrier reefs, that this channel or opening alway faced valleys in the included land.

The third great class are the Fringing reefs, these, so far as the coral reef itself is concerned, do not differ materially from the others, except that the encircling belt of coral is much narrower. Where the land slopes abruptly into the water the reefs are but a few yards in width, forming a mere ribband or fringe around the island, but when the slope is gradual, the width is much increased

extending sometimes as far as a mile from the land, and always to such a distance from the shore that the limiting depth of 20 or 30 fathoms is obtained, where the reef ceases. From the more flourishing growth of the outermost corals, the fringing reefs are usually highest at the outside, and the sediment washed inwards upon the reef, generally produces in the course of time, a shallow sandy channel. Such are the three great classes of coral reefs which are found scattered throughout the vast oceans, and principally in the tropical regions, but it must by no means be supposed that they are found indiscriminately united, on the contrary the atolls and barrier-reefs are never found in proximity to the fringing reefs. It has been remarked with surprise that while atolls are the most common coral structures throughout some vast portions of the ocean, such as the tropical Pacific and the Indian Oceans, they are entirely wanting, or very nearly so, in the tropical Atlantic and West Indian Seas, where the corals themselves, are exceedingly numerous. There is also another somewhat remarkable fact, that no single active volcano occurs within several hundred miles of a coral archipelago, or even a small group of atolls; and although most of the islands in the Pacific which are encircled by barrier-reefs are of volcanic origin, having remains of craters distinctly visible, yet not one of them is known to have been in eruption since the growth of the corals. In explaining by any theory the formation of coral reefs, we must consider all the phenomena presented by the three great classes as enumerated in the preceding description. To ourselves the explanation proposed by Mr. Darwin in his volume upon 66 The structure and distribution of Coral Reefs," is the most satisfactory, and may be briefly stated thus; islands, or a line of coast, being first skirted with fringing reefs, become atolls by a continual but gradual subsi

A

LEVEL OF SEA

dence of the land. Let us then take an island surrounded by

We

fringing reefs, and let this island with its reef, represented by the unbroken lines in the wood cut, slowly subside. As the island sinks down, the reef continually grows upward; as the island subsides the space between the inner edge of the reef and the beach becomes proportionally broader. A section of the reef and island in this state is represented by the dotted lines, A A, being the outer edges of the reef; C C, the lagoon; B B, the shores of the encircled island. This section is a real one (on the scale of .388 of an inch to the mile), through Bolabola in the Pacific. can now see why the barrier reefs are so far from the shores which they front. Supposing the island to still subside, the corals meantime growing vigorously upward, the last traces of land will finally disappear, and a perfect atoll be formed. We thus perceive why atolls so much resemble the barrier reefs in general size, form, and manner in which they are grouped together, for they are but the rude outlines of the sunken islands over which they stand. In proof of the foregoing simple and not at all improbable cause for the formation of barrier reefs, and atolls, Mr. Darwin gives some examples of actual subsidence now in progress, and also presents some evidence of the recent elevation of those islands and coasts which have fringing reefs. The sinking of the islands, or coast, for the formation of barrier reefs, or atolls, must necessarily have been very slow, and undoubtedly large archipelagos and lofty islands once existed, where now only rings of coral rock scarce break the open expanse of the sea; thus the only record left to us of the existence of vast tracts of land are the wonderful memorials of these busy architects; in each barrier reef we see evidence of land subsided, and in each atoll a monument of an island lost. Busy from the first ages of the world, when the primeval seas had but a few groups of living beings, of the lowest order of organization, the coral polype has toiled from day to day, and year to year, and is toiling now. What mighty changes have passed over our globe since that remote period in which the Geologist is first enabled to trace the existence of living beings upon the earth. How many tens of thousands of times the earth has revolved around the sun, and how many huge mountain chains of granite have been disintegrated, and their scattered frag

ments deposited in the deep bed of the ocean. Perhaps the foundation of some of our present coral islands, was begun in those remote ages, and that the successive architects of the solid pile, have reared a structure which has witnessed more than one revolution of the major axis of the earth's orbit.

We close with the following beautiful description of a coral grove, by Percival.

"The floor is of sand, like the mountain-drift,

And the pearl-shells spangle the flinty snow;
From coral rocks the sea-plants lift

Their boughs, where the tides and billows flow;
The water is calm and still below,

For the winds and the waves are absent there;
And the sands are bright as the stars that glow
In the motionless fields of the upper air.
There with its waving blade of green,

The sea-flag streams through the silent water,
And the crimson leaf of the dulse is seen

To blush like a banner bathed in slaughter;
There with a light and easy motion

The fan-coral sweeps through the clear deep sea;
And the yellow and scarlet tufts of ocean

Are bending like corn on the upland lea;
And life in rare and beautiful forms

Is sporting amid those bowers of stone,
And is safe when the wrathful spirit of storms
Has made the top of the waves his own.
And when the ship from his fury flies

Where the myriad voices of ocean roar,

When the wind-god frowns in the murky skies,
And demons are waiting the wreck on shore,

Then far below in the peaceful sea

The purple mullet and gold-fish rove,

Where the waters murmur tranquilly

Through the bending twigs of the coral-grove,"

ORGANIC REMAINS.

295

CHAPTER XII.

·Organic Remains.

"And thou didst shine, thou rolling moon, upon
All this, and cast a wide and tender light,
Which softened down the hoar austerity
Of rugged desolation, and fill'd up,

As 'twere, anew, the gaps of centuries."

Byron.

In the preceding chapters we have, though somewhat imperfectly, given a sketch of the great causes of change now in operation on our globe, and we have shown that the earth's surface has been, and still is, subject to perpetual mutations. What was once dry land is now the bed of the ocean, and what is now the bed of the sea will one day be elevated land. We have also seen that the crust or superficial covering of the globe is composed of strata succeeding each other in a well determined and regular order, and the remains of countless myriads of animals are entombed in them, which lived and died at periods long ante. cedent to the creation of the human race, nay, more than this, that almost every grain of sand and particle of dust wafted by the wind, teems with organized matter. We have lying before us specimens of whitish earth which to the unassisted eye appears but light chalky powder; we have but to wet a little of it and place it under the microscope and a thousand perfect forms are visible. From the midst of a lump of chalk we have extracted a nodule of flint, and by the hammer have chipped off several thin slices; one of these is now under the microscope by us, and we distinctly recognize two beautiful species of infusoria, as perfect and well defined as though now alive, and yet, these little beings have been entombed for myriads of years. What mighty changes have come over the face of our globe since the flinty sea encom