« ForrigeFortsett »
But if we do not take into account those spots where the depth is reduced from local circumstances, we may say, that, as a whole, the ship channel begins to the east, with a depth of about two fathoms between Fowey rocks and Soldier key, increasing gradually thence, until it reaches three fathoms between Pacific reef and Old Rhodes, then becomes again slightly reduced between Carysfort reef and Key Largo; after which, with the exception of the shoals between Pickle reef and Key Rodriguez, it deepens again to three, four, five, or even six fathoms, until, between Looe key and Pine islands, it shoals once more to fourteen feet. Farther on, it increases again to five, sis, and seven fathoms, the average depth between Key West and the reef being five or six fathoms; and still beyond, more towards the west, sinks to eight, nine, and ten fathoms between the western extremity of the Marquesas and the western end of the reef, where it spreads into the great depression separating the Tortugas from the Marquesas. The character of the bottom varies in different parts, as do also the living beings which it supports. Where it is the most shoal, as between Fowey rocks, Triumph reef, and Long reef, on one side, and Soldier key and the Ragged keys on the other, the bottom consists of coral sand, overgrown with what is called the country grass ; that is to say, a variety of the limestone algæ, mingled with Gorgonia, among which rise a number of coral heads.
To the west of Long reef, especially between Carysfort and Key Largo, the coral sand rises here and there, in the form of shoal sandbanks, intermixed with coral heads—an arrangement which is probably owing to the more rapid currents flowing in that part of the channel, which is precisely the turning-point of the direction of the reef. Such heads occur again about a mile and a half off Vermont key, half way between Key Tavernier and Indian key, outside of which Gorgonia and sponges are very abundant, upon a hard, white sand bottom. Similar heads are seen between Long key and Tennessee reef, and nearer the reef there are shoals of white coral sand, covered with Gorgonia ; but farther west, off Duck key, the bottom becomes softer. Off Bahia Honda, again, it is rocky—that is, studded with large heads, surmounted with soft, muddy sand. This change in the character of the bottom is more obvious westward, where the heads are fewer and the bottom more generally muddy, or covered with finer-grained sand. For instance, hard sand is observed between Loggerhead key and Saddle bluff; but nearer the reef, as far as the American shoals, we have soft mud, with shoals and coral heads. Off Boca Chica, the channel way has also a bottom of soft coral mud, while shoals, with coral heads, may be traced for three-fourths of a mile along the shores, as, again, towards the Sambos, in a depth of from three to two fathoms. The softness of the bottom in the vicinity of Key West, considered in connexion with the scarcity of coral heads in that region, shows that a soft mud formation is unfavorable for the growth of corals; and, indeed, this holds also good for the flats north of the keys.
A careful survey of all the varieties of rock occurring at Key West, as well as their peculiar superposition, had prepared us for a minute comparison between the keys and the main-land; but, nevertheless, we were no less surprised than delighted to find that the solid foundation of the main-land consisted of the same identical modifications of coral rocks which form the keys. Along all that part of the shore which was examined, as well as upon the shores of the Miami, we found everywhere the same coarse, oolitic rock, with cross-stratification, consisting of thin beds, dipping at various angles in different directions, precisely as we find it at the western extremity of Key West, excepting, perhaps, that the cross-stratification is here more prominent, the strata dipping more frequently in several directions within the same extent.
But it may be asked, what is the practical use of such detailed descriptions of the coral reefs for the coast survey? We need only allude to the universal impression of the dangers arising to navigation, from the growth of such reefs, to satisfy the most skeptical that a minute knowledge of the extent and mode of formation of those belonging to our own shores must be of paramount importance, were it only with reference to the position of light-houses. But there is another subject connected with this investigation, which is not less momentous. It is well known that in the Pacific, coral reefs have been raised above the levels at which they were formed by the agency of the living animals, and also that in other localities, sometimes in close connexion with those just mentioned, the ground is subsiding. These changes have been so often observed, whenever coral reefs occur, that the idea of subsidence and upheaval is naturally connected with the features of coral reefs, and the question at once arises, whether the reefs on our shores are thus undergoing variations of level, independently of their natural growth. We have seen how extensive are the changes produced merely by the normal growth of the corals, and the facts accompanying their increase. It now remains for us to ascertain whether this growth has taken place, or does at present take place, upon ground which has changed or is now changing its relative level in reference to the sea.
The facts already described afford a sufficient answer to the question. We are satisfied that as far as coral formations have been observed upon the main-land of Florida, and within the present extent of the coral reefs, no change of the relative level has taken place either by subsidence or upheaval of the coral ground, and that all the modifications which the reef has presented at successive periods have been the natural consequence of the growth of reef-building corals, with the subsequent accumulation of their products in the manner described above.
. . . . . . . .
There is in reality but one way of accounting for this equality of level in the successive reefs; which is, to suppose that their loftiest ridges are the maximum height at which materials can be accumulated by the natural agency of gales, and we have sufficient evidence to justify the adoption of this view.
The fact that, at present, the highest tides during the most severe gales do not reach the level of the bluff summits along the shores of the main-land, or even that of the maximum height of Key Largo or Key West, does not invalidate this supposition, for when the shore bluffs of the main-land were formed, the ocean had full sweep over the ground now occupied by the reef and mud flats, which did not then exist; and when Key Largo and Key West attained their maximum height, the outer reef did not yet form a barrier, checking the violence of the Gulf Stream in that direction. But, even with the present obstruction, we have evidence of the occasional rise of the water to heights which fully justify our assumption, that even the highest ridges on the shores of the main-land and on the reef have been formed by the action of severe gales. For, in the year 1846, the water rose eight and a half feet above high-water mark at Key Vacas. Key West was entirely inundated during the same gale ; and though that island is somewhat protected by the reef, even at present the rushes, driven upon it by the flood, may be seen among the trees and bushes, at a height almost equal to its loftiest summit. In 1841 the water rose ten feet above high-water mark at Cape Romaine, on the western shore of the peninsula.
These facts suffice to show that the explanation we have given of the formation of the reef is in accordance with the powers of the agencies to which it is ascribed, and, when taken in connexion with the peculiar arrangement of the materials of which they consist, seems to us to prove the justness of this view.
Physical Changes in the Gulf Stream.
There are several questions of the deepest scientific interest, which may be advanced by a due consideration of the facts observed upon the reefs of Florida. There we have a peninsula-a narrow, flat strip of land, projecting for about five degrees from the main land, between the Atlantic ocean and the Gulf of Mexico, and forming an effective barrier between the waters of the two seas, which otherwise, even by the change of a few feet in the relative level of the intervening peninsula, would communicate freely with one another; and this peninsula we now know to have been added to the continent, step by step, in a southerly direction.
We know that the time cannot be far behind us when the present reef, with its few keys, did not exist, and when the channel, therefore, was broader, and the Gulf Stream flowed directly along the main range of keys. We know, further, that at some earlier period the keys themselves were not yet formed, and that then the channel between Cuba and Florida was wider still, washing freely over the grounds now known as the mud flats, between the keys and the main land, and that there was then nothing to impede a free communication between the Gulf of Mexico and the Atlantic ocean. The channel of the Gulf Stream was not only wider-it was also less shallow along its northern borders, for the whole extent of soundings south of the main land of Florida was an uncovered coral ground, upon which the deep-water species were just beginning to spread. But we may trace the change farther. There was a time when neither the southern bluffs of the continent, nor Long key within the everglades, nor even the everglades themselves, existed'; when, therefore, the Gulf Stream had a broad communication with the Atlantic, and the southern shores of the United States extended in almost unbroken contiguity from west to east, from the shores of Texas and Louisiana to St. Augustine. At that time the gulf channel was, in reality, a broad bay, as broad as the gulf itself, destitute of all those obstructions which now cause the tropical current to follow such a circuitous course between the West India islands, through the Caribbean Seas, and around the peninsula of Florida. The influence which the Gulf Stream has upon the climate of the Atlantic is so well known, that its connexion with the changes which the current itself has undergone within a comparatively recent period cannot be overlooked. If it is true, as we have every reason to believe, that the temperature of the Gulf Stream, in connexion with the temperature of the southwesterly winds blowing obliquely across the Atlantic, modifies that of the western coast of Europe,-if it is true that the Gulf Stream and the southwest winds have an influence in determining the course of the isothermal lines upon the two sides of the Atlantic, and of raising beyond their normal altitude the mean annual temperatures of northwest Europe, then we may look to the physical changes which have occurred on the southeastern extremity of the North American continent for the cause, or at least a partial cause, of those changes of temperature which have taken place in the beginning of the present period, in those very northwestern portions of Europe which are now so much warmer than the corresponding latitudes on the American continent, and which, soon after the accumulation of the glacial drift, had as low mean annual temperatures as the coasts of Labrador, Nova Scotia, and New England in our day.
Changes in ages to come. Among the questions contained in your instructions, you ask whether the growth of coral reefs can be prevented, or the results remedied, which are so unfavorable to the safety of navigation. I may say that here, as in most cases where the operations of nature interfere with the designs of man, it is not by a direct intervention on our part that we may remedy the difficulties, but rather by a precise knowledge of their causes, which may enable us, if not to check, at least to avoid the evil consequences. I do not see the possibility of limiting in any way the extraordinary increase of corals, beyond the bounds which nature itself has assigned to their growth. We have seen how successfully several reefs have been formed, more or less parallel, within the limits of the peninsula of Florida, as well as beyond the main land. We have seen, also, how these parallel or concentric reefs have been gradually transformed into main land by the accumulation of coral sand and mud with other loose materials, and also that the keys are now slowly annexed to the main land, by the same process. We may therefore safely infer, that, as far as the conditions exist for the formation of similar accumulations of loose materials, they will continue to occur, but they will never extend beyond the natural foundation from which a coral reef may rise; and as we now have sufficient evidence that this foundation is a seabottom, under from 12 to 20 fathoms, we may be satisfied that outside of the present outer reef, where the slope is steep, sinking rapidly to unfathomable depths, there is no opportunity for the growth of a new reef.
Here and there the reef may widen somewhat, towards the Gulf Stream, within those limits at which the depth does not exceed twenty fathoms, and from the knowledge we already possess of the soundings outside the reef, we know positively that this is nowhere a broad stream; we may therefore rest assured that the changes which are going on will chiefly consist in bringing up the reef, for its whole extent, to the surface of the water, with occasional intervening channels kept open by the currents, such as exişt now between the keys; that this reef once matured, will be covered by coral debris, becoming transformed into a range of keys, similar to that which exists now inside of it; that the depth of the ship channel between the reef and the main range of keys will gradually lessen, and the channel itself be changed into mud flats, similar to those stretching now between the keys and the main land. In still more remote ages the present mud flats may become swamps, elevated above the reach of the tide-waters, like the everglades; and this process may perhaps be extended to the present ship channel. But unless some great revolution in nature modifies the present relative level between land and sea, it may safely be maintained that the present outer reef is the final southern boundary of the North American continent, and that the sooner a system of light-houses and signals is established along the whole reef, the better; for this is, after all, the shore which is to be lighted, and not the range of keys which is within the reef. In relation to the western range of keys, and the western extremity of the reef, we may expect, in course of time, to see the depression between the Marquesas and Tortugas gradually lessened by the increase of the reef, so that the westernmost group of islands may finally stand in as close, connexion with the keys more to the west as they now bear to each other, the passage between them being reduced io as, narrow a channel as Boca Grande, between the Marquesas and the Mangroves.
The shoals west of Cape Sable may, undoubtedly, also increase in extent westward; but how far the currents from the northwest may limit this accumulation, in connexion with the changes which the currents themselves may undergo by the increase of the keys to the west, it is beyond the power of human foresight to determine.
These practical results—for so we venture to call the general conclusions last presented—although they are purely scientific deductions from general principles, may satisfy the most obstinate supporters of the matter-of-fact side of all questions, of the advantages of scientific illustrations in the daily walks of life, and also justify the course which
alier of-fact side of may satisfy the