824 REPOBT— 1893. 



he a temporary or local variation caused hy exceptionally severe winters. The 

 limit of forest growth does not coincide with the isotherms of mean annual tem- 

 perature, nor with the mean temperature for January nearly so closely as it does 

 with the mean temperature for July. It may be said to approximate very nearly 

 to the July isotherm of 53° F. We may therefore assume that a six-foot blanket 

 of snow prevents the winter frosts from killing the trees so long as they can be 

 revivified by a couple of months of summer heat above 50° F. 



The limit of forest growth is thus directly determined by geographical causes. 

 In Alaska and in the Mackenzie Basin it extends about 300 miles above the Arctic 

 Circle, but in Eastern Canada the depression of Hudson's Bay acts as a vast ice- 

 house and the forest line falls 500 miles below the Arctic Circle, whilst on the east 

 coast of Labrador the Arctic current from Baffin's Bay sends it down nearly as far 

 again. On the other side of the Atlantic the limit of forest growth begins on tho 

 Norwegian coast on tke Arctic Circle, gradually lises until it reaches 200 miles 

 farther north in Lapland, is depressed again by the ice-house of the White Sea, but 

 has recovered its position in the valley of the Pechora, which is rather more than 

 maintained until a second vast ice-house, the Sea of Okotsk, combined with Arctic 

 currents, repeats the depi-ession of Labrador in Chuski Land and Kamchatka. 



There are no trees on Novaya-Zemlia. Two or three species of willow grow 

 there, but they are dwarfs, seldom attaining a height of three inches. Novaya- 

 Zemlia enjoys a comparatively mild winter, the mean temperature of January, 

 thanks to the influence of the Gulf Stream, being 15° F. above zero in the south, 

 and only 5° F. below zero in the north. The absence of trees is due to the cold 

 summers, the mean temperature of July not reaching higher than 45° F. in the 

 south, whilst in the north it only reaches 38° F. 



The Indians of Canada have discovered that when they want to find water in 

 winter it is easiest reached under thick snow, the thinnest ice on the river or lake 

 being found under the thickest blanket of snow. On the same principle the tree 

 roots defy the severe winters protected by their snow shields, but they must have 

 a certain temperature (above 50° F.) to hold tlieir own in summer. 



The influence of the snow blanket is very marked in determining the depths to 

 which the frost penetrates beneath it. Thus we find that a Norwegian writer, 

 alluding to latitude 62°, remarks ' that the ground is frozen from one to two and a 

 half feet in whiter, but this depends upon how soon the snow falls. Higher up 

 the mountains the ground is scarcely frozen at all, owing to the snow falling sooner, 

 and, in fact, if the snow falls very early lower down it is scarcely frozen to any 

 depth.' Similar facts have been recorded from Canada in latitude 63°. ' On 

 this prairie land, when there is a good fall of snow when the winter sets in, the 

 frost does not penetrate so deep as when there is no snow till late.' Another 

 writer a little further south, in latitude 51°, says: ' I am safe in saying that the 

 frost penetrates here to an average of five feet, except when we have had a great 

 depth of suow in the beginning of winter, in which case it does not penetrate 

 nearly so far.' 



It is not so easy to explain the boundary line between the forest and the 



STEPPE. 



There are two great steppe regions in the Polar Basin, one in Asia and the other 

 in America. The great Barabinski steppe in South-west Siberia stretches with 

 but slight interruptions across Southern Russia into Bulgaria. The great prairie 

 region of Minnesota and Manitoba reaches the Mackenzie IJasin, and outlying plains 

 are found almost up to the Great Slave Lake. The cause of the treeless condition 

 of the steppes or prairies has given rise to much controversy. My own experience 

 in Siberia convinced me that the forests were rocky, and the steppes covered 

 with a deep layer of loose earth, and I came to the conclusion that on the rocky 

 ground the roots of the trees were able to establish themselves firmly so as to 

 defy the strongest gales, which tore them up when they were planted in light soil. 

 Other travellers have formed other opinions. Some suppose that the prairies were 

 once covered with trees which have been gradually destroyed by fires. Others 



