Mr. J. Prestwich on the Drift Deposits. 65 



checked when the temperature had risen from the extreme cold to a 

 point 12|° below the present mean annual temperature. This would 

 reduce the mean annual temperature here to 37^°, — that of Moscow 

 and Quebec, with which the climate at the higher gravel period has 

 been before compared, being respectively 40° and 4 1'', — and would 

 agree with what has been considered the probable mean winter 

 temperature of that period, viz. one between 10° and 20°. 



Taking this as the starting-point, the effect of such conditions with 

 reference to the quantity of ice and snow stored up during this period 

 of cold, and to its effect on the river-discharges for many years after- 

 wards during the period of the valley gravels, has to be considered. 

 The melting of the winter snow would necessarily cause spring floods. 

 Another cause of floods is the fall of rain whilst the ground is still 

 frozen. These causes, combined possibly with a larger rainfall, must 

 have afforded to the old rivers, either permanently or, at all events, 

 during spring-time, a volume of water far exceeding any present sup- 

 ply, and given them more of a torrential character. Instances are 

 quoted from Sir R. Murchison's * Russia' and Wrangel's 'Siberia,' 

 and others, to show how this is still the case every spring in northern 

 countries, causing a rise in the rivers of from 10 to 40 feet, and 

 inundating the adjacent valleys. 



Other forces, however, besides an increase in the water-power, 

 seem required to account for the excavation of the great valleys ; and 

 the author thinks that cold and ground-ice have performed a very 

 important part in the operation. In support of this view, he adduces 

 the opinion of Arago and the observations of M. Leclercq and Col. 

 Jackson, both of whom show how constantly this ice is formed in 

 cold climates in rivers with stony and gravelly bottoms, such as the 

 old post-pleiocene rivers must have been. Amongst other obser- 

 vations given are those of M. Weitz, who states that in the north 

 of Siberia the formation of ground-ice can be seen in the rivers at a 

 depth of 14 feet and more, and that in "rising from the bottom 

 the masses of ice bring up with them sand and stones, and let them 

 down at places far distant from whence they came;" and he concludes 

 " 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 deposit 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." 



Another agent would co-operate with the last ; this is the freezing 

 of the ground and the rending of rocks by frost. Taking extreme 

 cases, Crantz shows to how great an extent this operates in Green- 

 land ; Dr. Sutherland gives some still more striking instances on the 

 shores of Barrow Strait, and Sir J. Richardson on the Mackenzie 

 River. Even in our country, the disintegration produced during one 

 severe winter on a fresh vertical section of chalk is very striking. 

 A remarkable instance is quoted from Sir R. Murchison's * Russia,' 

 of a long terrace of angular blocks of limestone broken up and left 

 by the winter-ice 30 feet above the summer level of the Dwina 

 near Archangel. 



Ann. ^ Mag. N. Hist. Ser. 3. Vol.x, 6 



