Erratic Blocks in Plains, 227 



characters, different from those which I have described as be- 

 longing to mountains and their valleys. 



Just as the erratic phenomenon is localized in the vicinity 

 and in the interior of mountains, so does it exhibit uniform 

 characters in the low country and in flat regions, covering 

 vast tracts whose limits cannot with precision be referred to 

 determinate centres. Blocks are seen extending from one 

 mountain-chain to another, across considerable depressions of 

 the surface ; the accumulations of blocks transported from one 

 place to another are no longer arranged in linear continuous 

 series as in the valleys, where they form mounds or ramparts, 

 which are moraines properly so called, but they are dispersed 

 irregularly over the surface ; the nature of the rocks mixed 

 together in these accumulations no longer indicates an origin so 

 limited as that of those moraines even which arc at the mouths 

 of the valleys. The dispersion of these blocks in different 

 countries has not hitherto been described with sufficient care, 

 and more particularly the erratic angular blocks with a rough 

 surface have not been sufficiently distinguished from those 

 that are rounded, polished, and scratched. There are, how- 

 ever, very important differences in this respect. In Switzer- 

 land, for example, we nowhere meet with large blocks, whe- 

 ther angular or round, whose surface is rubbed, polished, and 

 scratched with rectilinear striae, at great distances from their 

 origin. Whatever may have been the cause of the transport 

 of the erratic blocks of the Alps and the Jura, it always hap- 

 pens that the great mass of the large blocks have arrived 

 there with rough surfaces and well marked angles, and that 

 the pebbles of smaller dimensions alone are worn, rounded, 

 polished, and* scratched with rectilinear strife. We may 

 easily convince ourselves of this fact by walking along any 

 part of the Jura chain. Another peculiarity worthy of atten- 

 tion is, that with us the large angular blocks generally repose 

 on the more or less considerable masses of rounded and po- 

 lished pebbles, and that these latter often pass into a fine 

 sand or a clayey paste, which covers directly the polished sur- 

 faces of the solid rocks wherever the pluvial water, the melt- 

 ing snow, and the torrents resulting from it, have not caused 



