Obrázky stránek

The banks of the Ohio, like those of the Connecticut, and in fact we believe of most of our rivers, are composed of two or three platforms, or distinct plains, rising one above another by elevations varying from ten to fifty feet. These platforms are alluvial, extending from hill to hill across the valley through which the river finds its passage, and have evidently been deposited at some former period by the waters of the river. But it is common for the highest platform, or that bordering on the mountainous or hilly formation, to be sixty, eighty, and even a hundred feet above the highest level to which the river now rises. The necessary result is, that the quantity of water which flowed in the river, when these deposits were made, was such as to fill the whole valley to that elevation, and as the water for some cause became diminished in quantity, it gathered itself into a narrower channel, and left the dry soil on its margin. One of the most singular circumstances attending this diminution of the water, is, that it seems to have been done at certain periods, and thus to have formed the regular succession of ascents and plains mentioned above. Had the subsiding of the water been uniform, the surface of the deposit would now be a gradual slope, from the base of the hills to the margin of the river. As these platforms and ascents are commonly two or three in number, on the borders of the large rivers east and north of the Mississippi, and we suppose of the Rocky Mountains, it is reasonable to refer them all to a cause acting at the same time, and to draw the conclusion, that the northern continent of America has at two or three particular seasons undergone signal revolutions, either by internal convulsions, or by the sinking of a large extent of country now occupied by the Atlantic Ocean, and thus leaving a space to be filled by the waters of the valleys, which have since dwindled into comparatively small streams. There is a difficulty, however, about the whole matter, when we ask how these immense rivers were supplied in former times, since, by the ordinary process of nature in evaporation, rains, and internal circulation, no more water is produced than enough to sustain the rivers in their present diminished channels.

Dr Drake does not puzzle himself, nor his readers, with speculations on these general bearings of the subject, but his investigations are chiefly confined to the valley of the Ohio.

He first inquires into the cause of the great excavation through which this river runs, and gives it as his opinion, that this broad valley, as well as that of the Mississippi, was originally caused by the sinking of a region now covered by the Gulf of Mexico, and thus giving a southerly current to the waters, which before that time overspread the regions at the north. In this way the great valleys were formed by the strong currents and perpetual abrasion of the waters. Another mode of accounting for the same thing would be to suppose, that the elevations and depressions of the surface, or the mountains and valleys, were produced by some violent convulsion within, and that the waters sought a passage in the most depressed parts. From various geological appearances Dr Drake thinks it quite certain, that such could not have been the origin of the great valleys and elevations of the west, although he allows that on the south of the Ohio river, in some parts of Kentucky, there are ravines and abrupt eminences, which indicate the action of some violent cause beneath. In modern times the river has become contracted in width, and the bed of its channel deepened. This channel is now worn many feet below the bottom of the alluvial deposits on its margin. It is continually increasing in depth, though Dr Drake says very slowly, as the current at the bottom of the river is always much less rapid than at the surface, except at the time of floods or freshets. This is easily ascertained in the summer season, when the water of the river is clear, and the bottom can be distinctly seen, Accumulations of light particles are found resting there, when the current at the surface is so strong, as instantly to carry away much heavier substances. It follows that the attrition of the water at the bottom is much less, than would be

apprehended from its velocity above, and that the process of deepening the channel is extremely slow.

Dr Drake accounts for some remarkable geological appearances in the western country in a manner so curious and plausable, that we shall present his views in his own language. After describing the gravelly substances, or debris, which occur in the regions of the Ohio river, and which are found of a similar composition and character, as he says, all over tłie western and northern parts of the United States, he proceeds as follows.

We are hence, I think, justified in the conclusion, that its origin. was in the north, and that it was brought and deposited on the surface of this country by currents, which in ancient times flowed from beyond the Lakes to the Gulf of Mexico, and of which it may be regarded as the sign and the effect.

A more recent formation, than many of the alluvial beds contained within the limits just defined, is the stratum of foam spread over the surface of our hills and valleys in an overlaying position. This appears to be the same, that in the north of Xarope is denominated geest, and which Mr De Luc considers as the last deposit made by the sea before its final retreat.'

• The deposition of the geest seems to have been the last operation, which the waters of the north performed upon this region, and was of course subsequent to the excavation of the valleys, as no deposit could have remained upon their acclivities, while the agent which formed them continued its action. To this formation belong the great blocks of foreign primitive transition, and old floetz rocks, which have excited in travellers so much astonishment, and which, in one point of resemblance at least, approximate the region south of Erie, Huron, Michigan, and other lakes, so closely to that which stretches from the southern shores of the Baltic Sea.

• These masses in the neighborhood of this place, [Cincinnati,} are for the most part solitary, but in the interior of the State it is not uncommon to find them grouped into heaps, which are slightly covered with soil; and it is, I suspect, an aggregation of this kind, on one of the Islands of Lake Huron, that a British officer mistook for granite in place. The size of these masses extends from that of gravel and pebbles to the diameter of eight or ten feet. The larger blocks are frequently found upon the old alluvial plains, but never, that I have understood, within them. Their geographical range is over the same region with the smaller foreign debris of our valleys, but more limited to the south west. I have never seen a single block on the opposite side of the Ohio, and am not informed that any have been observed lower than the thirtyninth degree of latitude.

I do not entertain a doubt, that these fragments were enveloped in large fields of ice in a region far beyond the Lakes, and floated hither by the same inundations, that brought down and spread over the surface of this country the geest in which they are imbedded. In the southern parts of this formation they are not found; but this should be attributed to the influence of the climate. The ice, to which they were attached, could not of course pass a certain latitude; and from the great increase of these masses as we advance towards the north, it would seem that many of the icebergs suffered dissolution long before they arrived at this maximum. Future

observers will no doubt trace them to their parent strata in the arctic regions, as Von Buck has traced those, which are lodged on the shores of the Baltic. The ice islands of the Atlantic ocean may reasonably be supposed to bring down, and deposit on its bed in the Temperate zone, primordial masses similar to those spread over some parts of this and the European continent. These islands are, I believe, not often seen further south than the fortyfirst degree, near two degrees north of their southern boundary here. This is probably attributable to the Gulf Stream; but for which, the larger tracts of ice would undoubtedly attain as low a latitude as the southern limits of the primitive blocks in this country; and hence a probable conclusion may be drawn, that the temperature of the northern hemisphere has undergone but little change, since the remote epoch when this part of the continent was for the last time subjected to inundation.'

From these facts, and this mode of reasoning, it would seem, that at some former period the ocean flowed over this continent with a current setting from north to south, and that the present features of the earth's surface in these regions have taken their shape and character from the action of this cause. Mr Hayden has pursued the same inquiry, in regard to the rivers and soil on the Atlantic coast, and has come to the same result. The only additional proof, now required, is that mentioned by Dr Drake as having been applied by Von Buck. Let it be established, by observation, that the rocky fragments, deposited throughout the alluvial formation of this country, are of precisely the same kind as the primitive masses in the polar regions of the American continent, and the demonstration will approach a degree of certainty, which will be satisfactory to most minds.

The fifth article in the volume is one of a good deal of interest and value. It contains a series of observations, by Mr John Hamilton, on the Winds, Currents, the Gulf Stream, and the Temperature of the Air and Water in the North Atlantic Ocean. The tables are skilfully constructed, and the observations carry with them every appearance of accuracy. These latter were made during twentysix voyages across the Atlantic, chiefly between Philadelphia and Liverpool. It was Mr Hamilton's object to ascertain, whether any substantial results could be derived from a course of observations of this sort, which should be of uniform and permanent advantage to navigators. In summing up the matter, however, he seems not to be very sanguine, although he clearly shows, that a methodical use of the thermometer, in determining the temperature of the water, will enable the mariner infallibly to tell, after a little practice, when he is in a current, or on soundings. He says that currents at sea are usually from two to four degrees warmer, than the water out of the current. On soundings the water is always warmer than off, although the temperature varies more than in currents, as it depends much on the depth of the water, and the position of the coast. Along the American shore the difference of temperature varies also with the seasons, it being less in summer than in the other seasons. In navigating the Gulf Stream the thermometer is of essen

tial use.

The difference between the temperature on the north side of this current and the sea out of it averages ten degrees ; on the south side it is not so much, but never less than five degrees. In summer it is less than in winter. By knowing these data, and by using the thermometer three or four times a day, the navigator may always determine when he enters the Gulf Stream. It is not easy to define the exact limits of this stream, because it is more or less affected by the winds; and after it passes the Grand Bank of Newfoundland it is divided into several branches, the main current proceeding to the south east, and the others to the east and north east. There are counter currents on each side of the Gulf Stream, but Mr Hamilton thinks the temperature of these not to be perceptibly different from that of the stream itself. The thermometer, therefore, will not indicate the counter currents; but this is not of much moment, compared with the great utility of the instrument in determining the presence of the stream itself, which, on account of the variableness of the current produced by winds and other causes, is not easy to be ascertained by any mode of calculation. Mr Hamilton says, that many navigators who profess to use the thermometer, profit little by it, because they do not begin their observations till they suppose themselves approaching the Gulf Stream, or soundings, and then, if it happens that they have entered either, the temperature is changed from that of the ocean, and they have no means of an accurate comparison. To be of any use, the observations should be daily, and regular. Were every experienced and intelligent navigator to take the pains,

« PředchozíPokračovat »