516 ABSENCE OF DRIFT IN CERTAIN NORTH AND SOUTH ZONES. 
presents, indeed, a striking contrast to the latter, in its surface being covered with 
a coarse northern drift of a few feet in thickness, beneath which the upper beds of 
the Old Red Sandstone or the lower beds of the carboniferous limestone are occa- 
sionally detected. This distinction may be explained by the great difference of 
physical outline of the east and west banks of these two lakes ; for whilst those of 
Ladoga, at least for some distance from its southern end, — are flat and low, with- 
out the trace of a hard subjacent rock, those of Onega, consisting, as we have 
shown, of trappsean and quartz rocks on the west, and of hills of Old Red Sand- 
stone with its ichthyolites on the east (p. 47), form striking promontories, under the 
lee of which all this drift has been accumulated. It was also curious to observe, 
how exactly the line which this drift had taken, from north and by west to south and 
by east, was indicated by a change in the nature of its materials, as we traversed 
its direction and approached the city of Vitegra ; the granitic rocks common to all 
the region in the north still prevailing, whilst the boulders of quartz rock and trap 
disappeared. Their absence, again, is strictly coincident with the fact, that the 
eastern side of the lake to which we had advanced, contains no such rocks in situ 
at its northern end, bat simply Old Red Sandstone in its unaltered state. In 
ascending, however, the eastern sides of the lake we found fragments of Lydian 
stone and altered limestone, derived from masses which lie to the north of that 
parallel. In extending our researches eastwards to the plateaux on the banks of 
the Andoma and its tributaries, to examine the Old Red or Devonian rocks with 
their caps of mountain limestone (p. 74), we were much struck with the com- 
parative absence of drift. This contrast was, therefore, a proof, in addition to the 
many examples we have already cited, that the agency by which the distribution 
of the rounded and highly- worn drift had been accomplished, acted in north and 
south zones of greater or lesser width. Still, on these high grounds, an erratic 
northern block may here and there be detected, and on advancing to the southern 
or south-south-east slope of the carboniferous limestone, we again met with a pro- 
fuse spread of northern granitic boulders and blocks, but without a trace of quartz 
rock. 
For a considerable distance to the east of Vitegra, the country is singularly exempt from all northern and foreign 
detritus, the Old Red Sandstone being cither seen in situ, or the surface detritus being, as it were, entirely made 
up of that subjacent rock ; but on reaching the plateau which separates the waters flowing into the Dwina and White 
Sea from those which flow into the Baltic, the carboniferous limestone is occasionally seen to be covered with a drift, 
composed of northern boulders, mixed up with the limestone of the tract, — one of the largest of the granitic blocks 
near Perkina, resting upon the local detritus, was from twelve to fifteen feet in diameter. Again, all northern de- 
tritus either disappears or becomes exceedingly scarce, and the road to Cargopol for several stages is either formed 
