Imágenes de páginas
PDF
EPUB

And thus, ihe great mass of northern drift, inasmuch as no considerable part of its transfer can be accounted for by any minute causes or languid operations of water, is an irresistible evidence of paroxysmal action; and of action in a scale which may be judged of from the conclusion at which we have arrived :---an elevation of 45,000 square miles of sea-bottom through 500 feet. And this conclusion is equally certain, whether or not we suppose the machinery employed in the distribution of this mass froin the centre to be waves of translation. For the proposition that the laboring force expended in ihe transit of this mass of materials must be equal to the force exerted, and that this force must be exerted in such portions as, at every step, to overcome the “riction and tenacity of the masses of rock, shingle, and other detritus moved, is equally true, whatever be the machinery employed. As no gradual or minute action could move the masses in question through a yard of space, Do accumulation of such action, through any amount of time, could distribute the masses through the great distances which the northern drist has traversed, and spread them over the vast spaces which that formation occupies. The distribution of the northern drist belongs to a period when other causes operated than those which are now in action.

PostSCRIPT.-Perhaps it may throw some light upon the subject to remark that a wave of translation differs little from a " debacle” according to the notions of earlier speculators. A ware of translation is a debacle conceived according to the more exact notions to which modern science has led. Or rather, since a debacle was generally conceived as a vast torrent sweeping over the land, arising from the emergence of a submarine area, or some such cause, we may say that a wave of translation, in such cases as we have considered, is a debacle travelling along the sea after it has been shot off the land.

2. On the Slow Transmission of Heat through loosely coherent Clay and Sand; by JAMES NASMYTH, Esq.; communicated in a letter 10 LEONARD HORNER, Esq., P.G.S., (Lon. Quart. Jour. Geo. Soc., Aug. 1847.) When I lately had the pleasure to see you at the foundry, on drawing your attention to what appeared to me a remarkable example of the low capability of mineral substances for conducting heat, I was much gratified to find that you agreed with me in considering that the instance in view had an important bearing on several interesting geological questions, especially those relating to the theory of the central heat of the earth.

At your request I have much pleasure in sending you a statement of the instance in question, under the impression that it may chance to prove of some interest as an illustration of what may yet exist in respect to the state of the interior of the globe, as regards its high temperature.

The case, you will remember, was that of a large plate-iron pot, containing eleven tons of white-hot melted cast-iron-a temperature so high as to be quite beyond all thermometric certainty, but well-known to be the highest intensity of furnace heat, being quite equal to that of welding hot iron.

This vast mass of white-hot melted cast-iron, you will remember, stood in the pot for upwards of twenty minutes, and but for a thin coating of clay and sand of about half an inch thick, would have soon melted the bottom and sides of the pot.

This hall-inch thickness of mineral substance, however, was quite sufficient to prevent the conduction of the heat to the exterior; so completely so, that after this vast mass of hot iron had remained for upwards of twenty minutes in the pot, you could place your hand on the side of the vessel without feeling any inconvenient degree of heat ; and, as I mentioned to you, so slowly and imperfectly does this thin lining of half an inch of clay and sand permit the heat to pass outwards, that the entire mass inight rest there till it became cool ere the outside of the pot would have reached a temperature high enough to carbonize wood in contact with it, the radiation from the outside carrying away the heat as fast as the slow conducting power of the clay and sand lining transmits it.

So striking an instance of the low conducting power of such substances is not frequently met with, and it appears to me that it is calculated to remove some of the doubts occasionally expressed respecting facis which indicate a high temperature in the interior of the earth. If half an inch of mineral matter thus intercepts the communication of so high a temperature as that of a mass of eleven tons of white-hot castiron, what may not two or three hundred miles of similar substances effect, in preventing the central heat of the earth from developing its action beyond a very moderate extent towards the surface? If this reasoning be correct, it tends to show that there may exist below the crust of the earth, a mass of fluid molten matter which at depths of two or three hundred miles may have a temperature transcending all our ideas of high heat, the only indications of which, at the surface, are afforded by volcanos, hot springs, and that regular increase of temperature as we descend towards the interior found in deep mines, or by deep borings.

There are many other instances of this nature which I could bring forward, as exhibiting the remarkable non-conducting power of clay ; many such examples are every day before the eyes of our manufacturers who have to do with furnaces where intense heat is employed. The fire-brick lining of such furnaces is only from 41 to 9 inches thick, and yet while the heat within is as high as our furnace powers will carry it, ihe hand may be placed outside without suffering any inconvenience.

3. On the Changes of the Vegetable Kingdom in the different Geolog. ical Epochs ; by M. ADOLPHE BRONGNIART, (Edin. New Phil. Jour., Jan. 1848 ; from L'Institut, No. 714, p. 280.)— The changes which have taken place in the nature of living beings, since their first appearance on the globe till the period when the surface of the earth, having assumed its present form, has been covered by the creation which now occupies it, constitutes one of the most interesting departments of geology: it is the history of life and its metamorphoses.

The progress of modern geology presents to us the surface of the globe becoming renewed many times since the period when life first appeared upon it, under the influence of Creative Power. At each of these modifications-every time that a great bed of mineral matter covered a portion of the earth's surface, or a shaking of the crust of the globe wrinkled this surface, and produced new chains of mountains, the living beings which inhabited our earth, destroyed and buried in these sedimentary deposits, were replaced by a new creation more or less different from the preceding.

It would be a difficult task at this moment to fix precisely the number of these successive creations of animals and vegetables ; but science is every day leading us nearer to this result, although it requires more detailed facts to enable us to reach it.

At certain epochs, however, great changes in the physical state of our planet have been followed by modifications equally great in the nature of the beings which inhabit it.

These are the very decided changes which alone deserve our attention in the present instance ; for, on the one hand, they shew us each of the two organic kingdoms passing through varied forms, of which the different degrees are of great interest, owing to the remarkable order in which they succeed each other; and, on the other, the nature of the beings which correspond to each of these great geological periods, may afford us most valuable indications respecting the physical state of the earth, and its climate, at these different epochs, illustrative of the history of the formation of our globe.

From the most remote historical times, the vegetables inbabiting our globe have undergone no change. This is proved by the comparison of grains and plants preserved in the tombs of Egypt, with those which now grow in that country.

On the contrary, the plants of the latest geological periods,- those which occupied the earth before the last revolution of its surface, and whose remains are enclosed in the deposits named tertiary formations, --differ very considerably from such as now grow in these same places. They are, in general, species no longer existing in a living state, and their differences, relatively to the plants now living on the same ground, are greater as they occur in the more ancient beds of these tertiary formations. The most recent indicate a climate differing little from that of temperate Europe; the most ancient announce a warmer climate than now occurs in that region.

But in all these beds, which are very recent when compared with the other parts of the crust of the globe, we find vegetation, as a whole, agreeing in all its principal features with the mass of the vegetable kingdom which still inhabits the surface of the earth ; there are the same classes, the same natural families, often the same genera. The general characters of this extinct vegetation are the same as those of the existing vegetation, and we might suppose ourselves merely transported to another quarter of the globe. Viewed as a whole they are the same; the details only are different.

But if, on the contrary, we descend more deeply into the layers composing the earth's crust, and go back to the more ancient periods of the creation ; if we consider the vegetables preserved in the formations named secondary, which have preceded those of which we have spoken by many ages, we shall find the vegetable kingdom reduced to a much less considerable number of those natural groups which we name sami. lies or classes.

This variety of form and aspect, which gives such a charm to the existing vegetation, did not then exist; and, to characterize in a word the vegetable kingdom of those remote periods, we may say that the plants composing it, much less varied and numerous than those now covering our ground, were all deprived of what constitutes their greatest

SECOND SERIES, Vol. VI, No. 16, July, 1848. 16

ornament, namely, those flowers with brilliant envelops which belong to almost all the plants of our period. All the vegetables of the first geological periods were in fact analogous to our firs and serns, whose habit and elegant foliage form all their beauty.

In these ancient times of geological history, we may farther distinguish iwo great periods ; the one nearest our own times, during which terrestrial vegetation, almost entirely limited to three families, ihe ferns, Coniseræ, and Cycades, presented only species so far analogous in their most essential characters to those now existing, that they may be casily classified in the natural families I have just named; the other, more ancient, to which the vegetables belong that have produced great de. posits of coal, and numerous remains of which accompany beds of this combustible. The latter recede much more widely from actually living forms, enter with more difficulty into known families, evidently constitute other families altogether distinct from those of our actual creation, fainilies whose existence was not prolonged beyond this first geological period.

The singular organization and great dimensions of these first inhab. itants of our soil, have long thrown inuch obscurity over the great classes of the existing vegetable kingdom. Every day, however, the study of them is advancing, and now we can no longer doubt that these gigantic vegetables, so remarkable by their extraordinary sorms and by their structure, constitute special fainilies, allied, however, to the ferns and Coniferæ ; (that is to say, belonging to the great divisions of vascular cryptogams, and gymnospermous phanerogams.)

In conjunction with many true fernis, often arborescent, and with some Coniferæ, very different from those of our climate, these vegetables must have formed vast forests growing on a turly soil, produced by their detritus, and to which our coal owes its origin.

Thus, briefly to recapitulate ; during the earliest periods of the creation of living beings, the vegetable kingdom was composed only of plants belonging to the two classes of that kingdom distinguished by the simplest structure. These plants had ihen special forms, were of considerable dimensions, and the greater part constituted families now extinct.

At a later period, these two great classes still continued to exist alone on the earth, but their forms approached more to those which they present in the present vegetation ; the families peculiar to the most ancient epochs were already destroyed, and the numerous and varied families which were to appear in the tertiary epoch did not yet exist.

Lasily, during this laiter period, vegetation assumes characters analo. gous to those it now presents. The more perfect vegetables, known by the naine of angiospernious phanerogams, appeared in great numbers, and the vegetable kingdom is not distinguishable from that now existing but by characters of detail, or by differences analogous to those which diversities of climate still produce on the earth.

If we now compare the vegetables of the families which, like the ferns and Coniseræ, have been perpeluated during all the geological periods, from the most ancient up to the present, we perceive that such as belong to the most remote creations, are most allied to the plants of those families which now inhabit regions of the earth having a climate

very different from our own ; and that such, on the contrary, as we meet with in the most recent beds, become more analogous to the species which still grow in these same countries, as the geological period to which they belong approaches nearer our own.

Every thing, therefore, proves, on the one hand, that the different vegetable creations which have succeeded each other on the globe have become more and more perfect; on the other hand, that the climate of the surface of the earth has been greatly modified since the earlier times of the creation of living beings up to the commencement of the present epoch.

4. Geology anil Topography of the Isthmus of Panama ; by EVAN Hopkiss, C.E., F.G.S., (Mining Journal, April 8, 1848; translated from the Bogotá Gazette of the 9th Sept. 1817.)-By reference to a prepared plan, it will be observed that the Cordillera of mountains, forming the chain of union between the two Americas, is curved in the shape of an arc on the Isthmus of Panama--the convex side faces the north, the easterly portion runs in a south-eastern direction towards Darien, and the south-west prolongation extends to the shores of the Pacific, from whence it again takes a westerly turn towards Veragua. The westerly part of the curve, between the rivers Trinidad and Gorgona, is broken, and the continuity of the chain interrupted by the oblique intersection of the River Chagres. Towards the north-east, between the sources of the Boqueron and San Blas, the chain forms a broad mass of great elevation, and sends out numerous lateral branches, from which the rivers Chagres, Pequeni, and Boqueron take their origin, and also the Cascajal, which runs to the north. The old road to Portobello, which I took on crossing the Isthmus the second tiine, follows along the Buqueron principally on the bed of the river, owing to the rocky and precipitous nature of the banks, until nuinerous and deep waterfalls over basallic rocks, which exist near its source, render it necessary to leave the river, and travel along the steep sides of the surrounding mountains. The heads of this river consist of various branches, one of which arises from the break existing in the Cordillera, at an elevation of 700 feet, where the road across the mountain is carried ; and from the same point another stream runs to the north into the River Cascajal. In consequence of the great convolutions of this river, and the rugged and rocky nature of its bed, a day and a half was formerly spent in ascending it by boats from the point of union of the Pequeni to half its extent. The river and the road being synonymous terms in this route, our road continued descending as before, but rather in the waters of the Cascajal than along its banks, over the hard, slippery, sharp, and broken edges of the primary slates that traverse it, until we came to within a short distance of Portobelo. The great difficulties and obstructions, caused by inundations, which attend this route, and the choice made of such a direction, are a strong proof that the Spaniards found insuperable obstacles to the forination of a good road between Portobello and Panama.

From the pass in the mountain above alluded 10, towards ihe west, the chain is divided into numerous longitudinal branches; one proceeds from Portobelo, and terminates abrupily in the vicinity of this port, with an elevation of 600 feet; the central branch falls towards the bay of El Limon, and the southern branch forms the limit of the River Chagres,

« AnteriorContinuar »