DURABILITY OF GLACIATED SURFACES. 307 



regions thau elsewhere. Oxidation and decay of all kinds are slower than 

 under the influence of heat and the rapid growth of all the various lower 

 forms of plant and animal life. Not only are marks on rocks preserved in an 

 extraordinary manner in northern climates, but the great durability of timber 

 has been remarked by travellers in Norway and the Arctic regions of 

 Canada. Logs of such perishable wood as spruce, which even in this lati- 

 tude would disappear through decay in a few years, are found in a sound 

 state in the latter regions, where they have probably lain for thousands of 

 years. Even on the east coast of Hudson's bay I have recorded the occur- 

 rence of lines of drift-wood, principally spruce and cedar, on raised beaches 

 thirty feet above the highest tides, which would indicate a period of over 

 400 years, even if the rate of elevation were as rapid as my supposition of 

 seven feet in a century. 



The deposition of the thick sheets of till over the well-preserved grooved 

 surfaces at any given place could not have been quite contemporaneous with 

 the making of the grooves themselves, but must have required time. Again, 

 we should take into consideration the many things requiring great length 

 of time which have taken place since the till was left upon the surface of 

 the rocks, such as the submergence of the land and the deposition of various 

 stratified clay and sand formations upon it. At Ha-ha bay, on the Sague- 

 nay, the stratified clay of the Champlain formation, which overlies the drift, 

 has a thickness of upwards of 600 feet ; and in the valley at the head of Lake 

 Ontario the clay above the till is at least 200 and may be 400 feet thick. 

 The stratified gravels and sands of Burlington heights at this locality rise 

 107 feet above the lake, and are also sunk below it. These deposits lie upon 

 the stratified blue clay of the Erie formation, which in turn rests upon the 

 till. 



We cannot suppose that the change from the glacial condition to some- 

 thing like the present climate of North America was a sudden one. The 

 transition, whether brought about by astronomical causes or only from 

 changes in the elevation and distribution of the land and in the currents of 

 the ocean, must have been very slow. It is therefore very improbable that 

 the ice disappeared from all parts of the continent at the same time. There 

 must have been a gradual and progressive recession northward of the general 

 glacial condition, which may not yet have entirely ceased. Glaciers are 

 said to exist still in some parts of Baflinlaud. It is, however, more probable 

 that we have passed the period of greatest warmth, and that a colder con- 

 dition has again begun to creep upon us from the north. The continued 

 elevation in polar regions, historical facts in Greenland, the southward 

 retreat of the verge of the forests, and other circumstances favor this view. 



Southward of the central regions of dispersion it may be assumed in a 

 general way that the time which has elapsed since the disappearance of the 



