Boulder Formations and Erratic Blocks, 317 



regions. The masses of clay, sand, and gravel, are sometimes 

 of so great thickness that it is impossible to detect a trace of 

 the subjacent solid rock, over very wide tracts, even in the 

 beds of the Volga and the deepest cutting rivers. M. Du- 

 rocher, in his first memoir,* did not trace the erratic blocks 

 farther east than the forty-second degree of longitude, nor 

 farther south than the fifty-fifth degree of north latitude ; but 

 the authors of the " Geology of Russia" have described them as 

 extending 500 miles farther east, and above 200 miles farther 

 south. As the parent rocks of most of these huge fragments 

 are in Scandinavia and Finland, they have been, in some in- 

 tances, transported to a distance of 800 miles in a direct line.t 

 It is possible that the boulder formation may extend somevrhat 

 farther, but probably not much ; for there is reason to believe 

 that land on the east and south was above the level of the 

 sea, as has been already stated, at the time the country to the 

 west and north was submerged, which would stop the advance 

 of the boulder formation and erratic blocks, but in an irregular 

 line. No erratic blocks of northern origin have been seen for 

 a considerable distance westward of the Ural Mountains. 



There is a feature in the character of this superficial cover- 

 ing of detritus which is very important to attend to in tracing 

 its history, viz., that the materials are not always the same ; 

 that the principal mass in each district is of local origin, and 

 very clearly bespeaks its derivation to be in the subjacent 

 rocks ; and that the great northern drift is distributed in the 

 form of long sand-banks, " trainees^ or " osar,'' as they are 

 called in Sweden, often of great length and breadth, and ris- 

 ing sometimes more than 100 feet above the depressions be- 

 tween them, which last are occasionally of great width. These 

 trainees are often composed of finely laminated sand and clay, 

 containing shells identical in species with those now living in 

 the Baltic or in the northern seas ; they traverse, from the 

 shores of the Baltic, the Silurian, Devonian, and carboniferous 

 regions in succession, deriving new materials from each zone 



* Comptes Rendus, Janvier 1842. 



t Map accompanying " Geology of Russia." 



