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.