Essay on the Theory of the Earth

by baron Georges Cuvier

ON THE DISTRIBUTION OF BOULDER STONES IN SCOTLAND, HOLLAND, GERMANY, SWITZERLAND, AND AMERICA.

Numerous large blocks are met with in almost every country of Europe, and frequently far removed from their original situations. This is frequently the case in Scotland: thus, in the Edinburgh district, we have numerous blocks of primitive rocks, of which no fixed rocks occur nearer than in our Highland mountains.

In the north of Holland, Germany, and the countries bordering on the Baltic, enormous fragments of granite and syenite are scattered within certain limits. According to Humboldt, it seems to be now proved, that they have been carried southward, with a distribution like that of radii from a centre, from the Scandinavian peninsula, during some of the ancient revolutions of our globe, and that they have not originally belonged to the granitic chains of the Hartz and Saxony, which they approach without, however, actually attaining their basis[374]. Born, says Humboldt, on the sandy plains of the Baltic, and until the age of eighteen, not knowing any other rock than these scattered blocks, I could not but feel curious to know whether the new world presented any thing of a similar nature. I was surprised not to find a single block of this description in the Llanos of Venezuela, although the immense plains were immediately bordered to the south by a group of mountains entirely granitic[375], and which presents, in its broken and almost columnar peaks, traces of the most violent action[376]. Towards the north, the granitic chain of the Silla of Caracas and of Portocabello is separated from the Llanos, by a range of mountains which are schistose between Villa de Cura and Parapara, and calcareous between the Bergantin and Caripe. I was equally struck with the same absence of blocks upon the banks of the Amazon. La Condamine had already affirmed, that from the Pongo of Manseriche to the strait of Pauxis, not the smallest stone was to be observed. Now, the basin of the Rio Nigro and of the Amazon is also but a Llano, a plain like those of Venezuela and Buenos Ayres, the difference consisting only in the state of the vegetation. The two Llanos, situated at the northern and southern extremities of South America, are covered with gramineæ; they are Savannas destitute of trees. The intermediate Llano, that of the Amazon, exposed to almost continual equatorial rains, is a thick forest. I do not remember to have heard that the Pampas of Buenos Ayres or the Savannas of the Missouri[377] and New Mexico contain granitic blocks. The absence of this phenomenon appears general in the new world. It is probably equally so in the Sahara in Africa; for we must not confound rocky masses which pierce the soil in the midst of the desert, and of which mention has often been made by travellers, with mere scattered fragments. These facts seem to prove, that the blocks of Scandinavian granite, which cover the sandy plains on the southern side of the Baltic, in Westphalia, and in Holland, are owing to a particular debacle which proceeded from the north, to a purely local catastrophe. The old conglomerate (grès rouge), which covers a great part of the Llanos of Venezuela and of the basin of the Amazon, contains, without doubt, fragments of those same primitive rocks of which the neighbouring mountains are composed; but the convulsions of which these mountains present undoubted evidences, do not seem to have been accompanied with circumstances favourable to the transportation of great blocks. This geognostic phenomenon is so much the more unexpected, that nowhere in the world does there exist a plain so continuous, and which is prolonged with fewer interruptions to the abrupt declivity of a purely granitic cordillera. Before my departure from Europe, says Humboldt, I had already been struck with the observation that there are no primitive blocks in Lombardy, nor in the great plain of Bavaria, which is the bottom of an ancient lake, having an elevation of 250 fathoms above the level of the ocean. This plain is bounded on the north by the granites of the Upper Palatinate, and on the south by the alpine limestones, transition clay-slates, and mica-slates of the Tyrol.

Boulders, or loose blocks of alpine rocks, are found in the lower part of the Alpine valleys, which terminate in the great principal valley that stretches between the Alps and the Jura, from the Lake of Geneva to the Lake Constance; and are also found almost every where in this great principal valley. They are sometimes met with 4000 feet above the level of the sea, on the side of the Jura, facing the Alps, and also in considerable numbers in many of the valleys of the Jura itself. These blocks occur only on the surface, never in any solid rock, and no one ever met with them in the subjacent strata of sandstone, marl, or conglomerate of the hills and valleys, interposed between the Alps and the Jura; but they are sometimes found deep in the soil, or imbedded or surrounded with the debris formed by rivers.

The traveller is often surprised by the enormous magnitude of these loose blocks, some of them being calculated to contain 50,000 cubic feet. The smaller masses are distinguished from those brought down by rivers, by their position, that is, their occurring on heights and acclivities, where no river could ever have run. They may also be confounded with blocks from decaying conglomerate; hence it is proper to be on our guard, not only to distinguish these blocks from those derived from conglomerate rocks, but also from the rolled masses belonging to river courses.

The height at which they are found does not appear to have any relation to their magnitude, for we often find very large blocks at considerable heights, and also in deep valleys; and we also meet with small masses as well in the bottoms of valleys, as high up on the mountains.

They occur sometimes in heaps, or dispersed in single blocks; but these relations have no connection with their magnitude, because we often find large and small masses in the same heap, and single, large, and small, blocks on mountain summits, and in the bottoms of valleys. The smaller blocks are more or less rounded, but seldom so much so as the boulders of rivers, which have been exposed to long continued friction. The larger blocks are indeed angular, but not sharp edged. But in examining this relation, we must carefully distinguish whether or not the angles or edges are original, or have been produced by subsequent, natural, or artificial causes. Very often masses of this description are blasted with gunpowder, either with the view of clearing the fields, or of obtaining stones for building; and these, if left on the ground, may lead into error.

These blocks vary in their nature, some being of the primitive class, while others belong to those of the transition and secondary classes. In general, they appertain to rock formations, situated nearer to the central alpine chains than those of the places where they are found. Thus, no rocks of the transition class occur in gneiss valleys; no alpine limestone in transition valleys; and, in general, nowhere but in Jura, do blocks of Jura limestone make their appearance. Therefore, all the loose blocks of rocks between the Jura and the Alps, belong to the strata of the high chains of the Alps.

But these blocks have different characters in different districts. The loose blocks which occur in the river basin of the Rhone, and the Lake of Geneva, are quite different from those which lie strewed about in the river basin of the Rhine. These, again, are equally different from the loose blocks of the river basin of the Aare, as those of the Aare are from the blocks of the Lake of Zurich, and the valley of Limmat; and these in their turn are equally well distinguished from the great accumulations in the valley of the Reuss. It rarely happens that intermixtures take place among these different accumulations of debris, and this is a circumstance which must be attended to in our investigation.

It results from an accurate comparison of these loose blocks with those mountain rocks which occur in extensive chains in the high Alps; that the loose blocks of every known river basin agree with the rocks which form the sides of the upper parts of those high Alpine valleys, which are in immediate connection with these great water basins. Thus the loose blocks of the water basin of the Rhine are similar to the rocks of Bundten. We find in the Lake of Zurich, and in the Limmat valley, the rocks of the Glarner land in loose blocks. The debris in the basin of the Reuss consists of rocks of the mountains from which the Reuss takes its rise. The loose blocks of the water basin of the Aare are similar to the mountain rocks of the high Alps of Bern; and the loose blocks, found in the course of the Rhone, occur in fixed rocks in the Vallais.

It thus appears that the loose blocks are by no means irregularly dispersed over the great valley between the Alps and the Jura, but are distributed in the direction of distinct water basins. It also appears, that the loose blocks are not irregularly distributed in these different basins; on the contrary, that, in some parts of the basin, they are accumulated in great numbers; in other places they are rare, and in some situations none occur.

From the preceding observations, we may obtain some hints of importance in respect of the cause of this remarkable phenomenon. These loose blocks already occur in the alpine valleys, which open into the great valley, between the Alps and the Jura. They are found more abundantly in the wide parts of valleys immediately below the narrow or contracted passes, and few occur in the narrow, steep, and rocky parts of the valleys.

Loose blocks are found, at a greater or less height, in the smaller lateral valleys that open into the transverse alpine valleys, which terminate in the great valley between the Alps and the Jura. If these lateral valleys form passes (which lead over into other valleys by a lowering of the high mountain chain), which are not more than 4000 feet above the level of the sea, loose blocks occur, not only in these passes, but also more or less widely distributed in the opposite valleys. In the great principal valley which stretches between the Alps and the Jura, from the Lake of Geneva to beyond the Lake Constance, we find these loose blocks dispersed over all the hills whose elevation is not more than 3000 feet above the level of the sea; but even here the distribution of the blocks is not entirely irregular. The largest are found on such hills and acclivities as are opposite the mouths of the alpine valleys, in the great principal valley. The blocks are frequently found higher on such acclivities, than on the sides of those valleys which may be considered as a continuation of the alpine valleys. The loose blocks are found every where on that acclivity of the Jura range which is opposite to the Alps, and they are found highest and largest in those places which are directly opposite the mouths of the alpine valleys. In such places, the blocks again attain an elevation of nearly 4000 feet above the level of the sea; whereas, in the intermediate places, which are most remote from the places opposite the mouths of the alpine valleys, the blocks seldom reach at a height of 2000 feet above the level of the sea.

In those places where the Jura chain branches into the great valley between the Jura and the Alps, loose blocks are found in the valleys behind the projecting chains. The Jura range is sometimes intersected in places opposite to the Alps; and it is remarked, that loose blocks are met with in the valleys behind these intersected portions of the range; and that, when loose blocks occur in the Jura range, at a distance from the Alps, it is only in such places as are directly opposite to the intersected portions of the chain opposite to the Alps.

The circumstance of the non-occurrence of these blocks in the sandstone, marl, and nagelfluh, which occupy the great valley between the Alps and the Jura, proves that that revolution of our globe, by which these were dispersed, took place after the formation of these rocks, and may therefore have belonged to one of the latest changes which have contributed to the present form of the earth’s surface.

When we compare the relations of the alluvium of the rivers in valleys with those of the loose blocks, their similarity must strike every one. Thus, rolled masses are seldom deposited in those places where a river forces its way through a narrow passage; but where an expansion takes place, owing to the distance of the banks increasing, the rolled masses are sometimes accumulated in whole banks. The same loose blocks seldom occur in the narrow passages of the transverse valleys in the Alps; but as soon as widenings of the valleys take place below these narrowings, the blocks occur in abundance.

If, during a flood, a rupture takes place in the banks of a river, where it is contracted, a part of the stream will flow out by the lateral opening, and carry along with it rolled masses, even when the opening in the bank does not reach to the bottom of the bed of the river; for the mountain stream, loaded with boulders, carries them not merely in single masses along its bottom, but the flood-water of the stream generally attacks large sandbanks, or older beds of rolled masses, and carries along with it, accompanied with a terrible noise, whole masses, forces them over the lower banks, or through the chasm in the bank, and often deposites them several feet high, on an immediately succeeding widening of the river’s course.

In the same manner, we observe loose blocks deposited on high situations in the lateral valleys of the great transverse valleys, and dispersed over the passes into the neighbouring valleys. The height of the lateral deposites of loose blocks, and their position in the passes, and their passing into neighbouring valleys, are facts which assist us in judging of the extent of the power that may have acted during their transportation.

The striking agreement observable in the phenomena of the distribution of the loose blocks from the interior Alpine valleys to the interior valleys of the Jura, with those in the rolled masses carried along by rivers, must lead every one, who reflects on this interesting phenomenon, to the hypothesis, that these blocks may have been deposited in their present situations by an overwhelming flood, which burst from the Alps. It is true that this opinion is liable to many objections; but still it contains a more plausible explanation of the phenomenon than any other with which we are acquainted.

The loose blocks, in the different river-districts, being in general separated from each other, or if any intermixture takes place of the rolled masses of one valley with that of another, it being only on their edges, it is highly probable that the floods which burst from these valleys, and carried along with them the masses of rocks, may have been simultaneous, by which the flow of the one basin would bound and limit that of the other, and thus prevent the water-flood of one basin flowing into the neighbouring ones.

The contemporaneous occurrence of these different floods from the Alpine valleys, can alone, on this hypothesis, explain why this aqueous flood was so generally and so highly accumulated in the great valleys between the Alps and the Jura, as to reach the height of most of the sandstone mountains, and to a great elevation in the Jura, where many blocks are found deposited. But if the contemporaneous occurrence of these floods is proved by the facts already enumerated, to what cause are we to refer this simultaneous bursting of floods of water from so many Alpine valleys?

We observe, on the north-western side of the chain of the Alps, numerous openings, which, by their structure, seem to point out the action of violent floods. Let us suppose the numerous valleys, in the districts already described, closed at their present entrances, or openings, as would seem from their structure to have been formerly the case; the consequence of this arrangement would be the filling of the Alpine valleys with water, to the height of the lowest passes among the mountains, and thus an enormous accumulation of water would take place. This great body of water, if let loose at once, by the bursting of the lower extremities of the valleys, would form a flood which would sweep across the sandstone mountains, between the Alps and the Jura range, and even ascend high on the Jura itself. This flood of water, moving, probably, at the rate of 200 feet in a second, and loaded with debris of rocks, would carry masses, even these having a magnitude of 50,000 cubical feet, some thousand feet high, on the Jura range[378]. Geologists maintain, that the blocks or boulders met with in other countries, and arranged as those in Switzerland, have been deposited where we now find them, by the bursting of lakes; while those found on the shores of the Baltic, are conjectured to have been transported by a great rush of water caused by the sudden elevation of the land of Scandinavia. Another opinion has its advocates, which maintains that these boulders have been spread over different countries by the waters of the deluge.

Note F, [p. 26.]