294 
ME. PRESTWICH ON THE GEOLOGY OF THE DEPOSITS 
ness of 3| feet. He found the temperature at top and bottom not to vary one-sixth of 
a degree*, and that a “flaky congelation” forms in immediate contact with the bed of 
the river and “ becomes gradually transformed into solid ice, which, if not thawed in 
the spring at the bottom itself, gets detached and rises to the surface.” In noticing a 
paper by Dr. Plott, he observes “ that the flakes of ice which rise from the bottom of 
the Angara (in Siberia) often bring up in like manner large stones.” 
In a subsequent memoir f Colonel Jackson translates some interesting observations 
made in Siberia by M. Weitz, superior officer of the Mining Corps. They are not so 
exact as the observations of M. Leclercq, but they are so important, as showing the 
effects of such an agent under more favourable conditions of temperature, that I give 
the greater part of the extract. M. Weitz remarks that “ the great transparency of these 
rivers (of the North of Siberia) enables us to see clearly what is at the bottom. At a 
depth of 14 feet and more one might see the ice formed at the bottom, whose greenish 
tinge gave it an appearance somewhat similar to that of patches of the confervoidese 
It frequently happens that these pieces in rising from the bottom bring up with them 
sand and stones, which are thus transported by the current When the thaw sets 
in, the ice becoming rotten, lets fall the gravel and stones in places far distant from those 
whence they came So long as the congealed masses continue small with regard to 
the volume of the water immediately above them, they adhere as if rooted to the bottom ; 
but when by degrees they increase in bulk, the difference in their specific gravity 
operates to overcome their adhesion to the bottom, and they rise, bringing with them, 
as we have said, such gravel and stones as we find attached to them, whence we may 
conclude that not only does the current occasion a change in the bed of the river by its 
erosion of the looser soil which it carries from one place to depose in another, but that 
the ice which forms at the bottom of rapid rivers in very cold countries, tends also to 
effect a change in the beds of those rivers.” 
Colonel Jackson, it is true, thinks thatM. Weitz attributes too much influence to the 
bottom-ice in effecting changes in the beds of the rivers ; but the Neva, where Colonel 
Jackson’s own observations were made, is a deep muddy-bedded river, offering precisely 
the least favourable conditions for the formation of ground-ice. 
The interesting narrative of Baron Wrangell contains amongst much important 
scientific observation the following remarks: — “In theAnini, as well as in all the more 
rapid and rocky streams of this district, the formation of ice takes place in two different 
manners : a thin crust spreads itself along the banks and over the smaller bays where 
the current is least rapid ; but the greater part is formed in the bed of the river, in the 
hollows amongst the stones, where the weeds give it the appearance of a greenish mud. 
* Colonel Jackson found that the water at the bottom of the river was generally a fraction of a degree (■!■) 
above freezing-point when congelation commenced, an observation since confirmed by Mr. Adie in a recent 
communication to the Chemical Society (Proc. vol. xv. p. 90). 
f “ On Ground-ice in the Siberian Rivers,” Journal of the Royal Geographical Society, p. 417-18, vol. vi. 
1836. 
