of very large areas of this compact white limestone, and also for others of impure or argillaceous limestones.

Besides the kinds of coral rocks above mentioned, there are also the Beach and Drift Sand-rocks, which are accumulated and consolidated above low-tide level. These formations illustrate the common mode of origin of oolitic limestones. They also afford numerous examples of the formation of coarse and fine conglomerates consisting of beach pebbles— these pebbles being either worn corals, or shells, or sometimes of other kinds, if other rocks are at hand.

The general slope of the beach sand-rock and oölite, and the mixed stratification of the drift sand-rock, are identical respectively with those of beach and drift-sand deposits in other regions.

II. BEDS OF LIMESTONE WITH LIVING MARGINS.

The coral reef as it lies at the water's level is in fact a bed of limestone with living margins; and the living part furnishes material for its horizontal extension outward, and also, if a slow subsidence is in progress, for its increase upward. It illustrates an ordinary mode of formation of coral, or of shell, limestone, whatever the age.

III. MAKING OF THICK STRATA OF LIMESTONE.

The coral reef-rock has been shown to have in some cases a thickness of at least 2,000 feet (page 156.) The reefs are, therefore, examples of great limestone strata, nearly as remarkable in this respect as the largest of ancient times.

IV. SUBSIDENCE ESSENTIAL TO THE MAKING OF THICK STRATA.

The coral island reef-rock has been shown to depend for its thickness on a slowly progressing subsidence (p. 263).

This is the only method by which any thick stratum of limestone could be made out of a single set of species, for all such species have a narrow range in depth; and the only way, from any succession of species, if those species are alike in range of depth.

In the case of existing coral reefs, there is yet no evidence that the species of the lower beds differ from those of the top. There is also no evidence, in any part of any ocean, that there is a set of cold-water corals fitted to commence a reef in deep water and build it up to such a level that another set of species may take it and carry it up higher; the facts thus far gathered are all opposed to such an idea. Should it be hereafter proved that the corals of the inferior beds differ in species from those now existing, it will probably be found that the predecessors of those now living were also shallow-water species; so that the subsidence in any case was necessary.

V. DEEP-SEA LIMESTONES SELDOM IF EVER MADE FROM CORAL ISLAND OR REEF DEBRIS.

This point has been discussed on pages 143, 211. The facts show that the sediment or débris from a shore is almost wholly thrown back by the waves against the land where it originated, or over its submerged part in the shallow waters, and that it is not transported away to make deep sea formations.

The facts have also a wider bearing, for they teach that lands separated by a range of deep ocean cannot supply one another with material for rocks. The existence of an Atlantic ocean continent-an Atlantis-has sometimes been assumed in order to make it a source of the mud, sand and gravel, out of which the thick sedimentary formations of the Appalachian region of North America were made. But if this Atlantis were a reality, there would still have been needed, in addition

to the presence of such an ocean continent, a set of freight carriers that could beat off the waves from their accustomed work, and push aside the ordinary oceanic currents; or else Atlantis would get back all its own dirt.

VI. ABSENCE OF FOSSILS FROM LIMESTONE STRATA.

Absence of fossils has been mentioned as a frequent characteristic of the fine compact coral reef-rock, and also of the beach and drift sand-rock or oölite (pp. 153, 194). The rocks are formed at the sea-level, and in the midst of abundant life, and yet trituration by the action of the waves and winds has in many places reduced all to the finest material, so that an embedded shell is seldom to be found in the beach or drift oölite, and rarely too in much of the fine-grained coral reef-rock.

The interior basins appear to be eminently the place for making these non-fossiliferous limestones. This is the case in two widely different conditions: first, over the portions that are below the coral-growing depths, which are sometimes of great area; and second, in lagoons that have become so small and shallow that corals and large shells have all disappeared, and the trituration is of the finest kind, producing calcareous mud; such lagoons being properly in a marsh condition. These last appear to illustrate on a small scale the conditions under which many of the ancient non-fossiliferous, or sparingly fossiliferous, limestones were formed.

VII. THE WIDE RANGE OF THE OLDER LIMESTONES NOT EXEMPLIFIED AMONG MODERN CORAL-REEF FORMATIONS.

Coral-reefs, though they may stretch along a coast for scores of miles, are seldom a single mile in width at the surface; and if elevated above the sea, they would stand as broad ramparts separated by passages mostly 20 to 200 feet deep, and often of

great width. The substratum, however, is, in general, continuous coral-rock; and if these more elevated parts were removed by any process, after an elevation, they would leave a nearly level area of coral limestone often as extensive as the whole reef-grounds. In an island like Dean's, one of the Paumotus, these reef-grounds are 1,000 square miles in extent. Still greater are the Bahama banks, the largest of them being 350 miles long and 200 wide, an intervening "tongue of the ocean" excepted.

But the most extensive reef-grounds of the oceans are after all of small breadth compared with many of the ancient limestones of the continents; and the reef-rocks also are peculiar in their very abrupt limits, the margins sometimes descending at a steep angle a thousand feet or more. These differences between the new and the old arise in part from the fact that the coral reefs of the present era are made about small oceanic lands, or along the edges of the continents, while the limestones of ancient time were generally formed over the broad surface of a continent as it lay slightly submerged.

The Abrolhos reefs of the Brazilian coast, described on page 140, illustrate one of the methods by which the coral banks extend and finally coalesce into beds of wide extent; but these are small compared with the great limestones of early time, and owe their slight approximation to them as regards extent to the wide range of shallow waters there afforded. These Abrolhos reefs differ from most limestone beds also in being formed largely of the corals in the position of growth.

The tendency of modern reefs to grow up to the surface in narrow banks, separated by channels, appears to be unlike any thing we discover in the old rocks; and it seems to be an unavoidable result of growth in the sea, where the waves pile up barriers, and the currents make, and keep open, channels. The case of the Australian and Feejee reefs are good examples It

is possible that such barriers may often have existed in ancient time, and have disappeared through subsequent denudation of the surface. But may not the difference between the great even layers of the continental formations and those of a coral island have proceeded from the difference in the depth of the seas? Over the great shallow continental seas where the limestones were in progress, the waves may have generally been feeble, and therefore there may have been a less tendency to form narrow barriers and deep intervening channels.

The marsh condition of a drying-up lagoon with its forming limestones has been compared above with that under which ancient unfossiliferous limestones were made. The narrow limits of the former make the comparison unsatisfactory; for, in the coral island, coarsely fossiliferous beds are all the while forming about the exterior of the island, but a few miles at the most from the lagoon-marsh; while the ancient limestones retain their unfossiliferous character often through many thousands of square miles. Still, the above mentioned difference between the continental sea and the existing deep oceans may perhaps account for the diversity of results.

VIII. CONSOLIDATION OF CORAL ROCKS.

All true coral-reef rocks are examples of the consolidation of material mainly of coral origin—either mud, sands, fragments, or standing corals, the last with mud or sands intermixed-by (1), an under-water process; (2), at the ordinary temperature; and they exemplify the mode in which all other submarine limestones of organic origin have been consolidated. The process appears to depend on the presence (proved by chemical analysis) of carbonic acid in the sea waters that bathe and penetrate the sands. This carbonic acid is derived from three sources: from (1), the rains which wash it down from