Essay on the Theory of the Earth

by baron Georges Cuvier

Note E, [p. 23.]

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.