away on the wings of the wind water, but its surf. The long lifts itself up in water, and, curling cataracts of white foam, ar like deep thunder. form a ring round mounnce of from two to three miles on the outside from a great depth, the land by a channel two or three We have a fine example in Tahiti, in their t of the Society group. for f from Thus 15 are numerous in the Pacific Ocean; the largest mour 40 to 90 miles in their longest diameter, they enclose lagoons from 15 to 49 fathoms In almost every instance, an atoll contains an and deep. enter opening on the leeward side through which vessels may and find shelter. This fact, coupled with the circular form of these islands, at first led to the belief, that those coral formations were built upon the craters of submarine volcanoes; but this theory would plainly not account for the formation of fringing and encircling reefs, or of such a vast structure as the great barrier-reef of Australia. The theory, therefore, proposed by Mr Darwin, is now generally accepted. Having noticed that the coral reefs extend often to unknown depths, and knowing the reef-building corals rarely live at a depth exceeding 20 fathoms (120 feet) he was irresistibly led to the conclusion, that a great part of the bed of the South Pacific must be gradually subsiding. This fact seems to explain at once the various coral formations. In the first place, the coral polype would commence to build near the border of some island, and would thus form a fringing reef. If the island remained "y, the reef might increase in breadth, but would in a fringing reef. If, however, the island were bside, and the whole colony of zoophytes 't work, there would gradually be a clear between the reef and the island, and we xample of an encircling reef. If the inued, the central island would in time. and an atoll would be formed. reef-building coral cannot thrive except in clear water; the weakest part of the reef is therefore on the lee side, where the water is less clear; here in most atolls is found a wide opening which connects the central lagoon with the sea outside, and admits the flux and reflux of the tide. "Physical Geography," by W. LAWSON. A GREAT WATERFALL. PEOPLE who have never seen a really great waterfall are apt to despise waterfalls altogether. Those who have watched one know that the greatest exhibition of persistent natural energy in all creation is a great spectacle. Three years ago appeared a new claimant for the title of "the highest waterfall in the world." Niagara is, of course, unapproachable in its own class, and it is in the highest degree unlikely that any rival to it, for volume of water and breadth of fall, yet remains to be discovered. But in height, as is well known, Niagara stands low among great waterfalls, and the highest of those which can really claim to be great may yet reward the enterprise of some future African or South American explorer. Kaieteur, the waterfall which was discovered in 1870, is a fall of one of the western branches of the Esquibo, reef does not rise to the surface of the water, but its presence is marked by a tremendous surf. The long ocean swell, impeded by the barrier, lifts itself up in one continuous ridge of deep blue water, and, curling over, falls on the reef in dazzling cataracts of white foam, accompanied by an unbroken roar like deep thunder. Encircling reefs generally form a ring round mountainous islands, at a distance of from two to three miles from the shore, rising on the outside from a great depth, and separated from the land by a channel two or three hundred feet deep. We have a fine example in Tahiti, the largest of the Society group. Atolls are numerous in the Pacific Ocean; the largest are from 40 to 90 miles in their longest diameter, and they enclose lagoons from 15 to 49 fathoms deep. In almost every instance, an atoll contains an opening on the leeward side through which vessels may enter and find shelter. This fact, coupled with the circular form of these islands, at first led to the belief, that those coral formations were built upon the craters of submarine volcanoes; but this theory would plainly not account for the formation of fringing and encircling reefs, or of such a vast structure as the great barrier-reef of Australia. The theory, therefore, proposed by Mr Darwin, is now generally accepted. Having noticed that the coral reefs extend often to unknown depths, and knowing the reef-building corals rarely live at a depth exceeding 20 fathoms (120 feet) he was irresistibly led to the conclusion, that a great part of the bed of the South Pacific must be gradually subsiding. This fact seems to explain at once the various coral formations. In the first place, the coral polype would commence to build near the border of some island, and would thus form a fringing reef. If the island remained stationary, the reef might increase in breadth, but would still remain a fringing reef. If, however, the island were slowly to subside, and the whole colony of zoophytes still to keep at work, there would gradually be a clear channel of water between the reef and the island, and we should have an example of an encircling reef. If the process still continued, the central island would in time disappear, and an atoll would be formed. The reef-building coral cannot thrive except in clear salt water; the weakest part of the reef is therefore on the lee side, where the water is less clear; here in most atolls is found a wide opening which connects the central lagoon with the sea outside, and admits the flux and reflux of the tide. 66 'Physical Geography," by W. LAWSON. A GREAT WATERFALL. PEOPLE who have never seen a really great waterfall are apt to despise waterfalls altogether. Those who have watched one know that the greatest exhibition of persistent natural energy in all creation is a great spectacle. Three years ago appeared a new claimant for the title of "the highest waterfall in the world." Niagara is, of course, unapproachable in its own class, and it is in the highest degree unlikely that any rival to it, for volume of water and breadth of fall, yet remains to be discovered. But in height, as is well known, Niagara stands low among great waterfalls, and the highest of those which can really claim to be great may yet reward the enterprise of some future African or South American explorer. Kaieteur, the waterfall which was discovered in 1870, is a fall of one of the western branches of the Esquibo, the Potaro, or Black River. It was discovered by Mr Charles Barrington Brown, the surveyor to the British colony of Guiana, and ascertained by him to be 130 yards in breadth, between 20 and 30 feet deep just above the fall, and no less than 750 feet high, or four times the height of Niagara. For its surpassing beauty, the weird remoteness of its situation, the striking character of its approaches, and the exceeding difficulty of access to it, I am one of the half-dozen travellers who can answer. In consideration that no other waterfall of equal or greater height (clear leap) is of anything like the same magnitude, we announced it with confidence as "the biggest waterfall in the world." The waterfalls of the Yosemite Valley in California, which in some American accounts reach astonishing dimensions, have never had the luck to be seen by an Englishman in volume anything like the proportions of a "river set upright." Mr I have now the satisfaction, mingled with mortification, of announcing that Kaieteur has been outdone. Brown, in the last of his adventurous series of journeys to the sources of the great rivers of British Guiana, has discovered, from a spot near the head waters of the Massarund, what at 30 miles distance appeared to be an immense river descending bodily from the north-western face of the great precipice of Reraima mountain, "the attic story of the world." This extraordinary cliff is known to be 2000ft. in height, and appears inaccessible on all sides yet surveyed. The summit is flat, and of great extent. The fall is believed, on Indian authority, to belong to the Caruni River, a tributary of the Orinoco; and will be, therefore, in the territory of Venezuela. After tumbling sheer down that astonishing wall, the water rushes down a glacis1 of (perhaps) 3000ft. more, at an angle which cannot be less steep than 45°. |