CONDITIONS OF PEESENT GLACIATION DISCUSSED. 319 



In the first chapter of this vohiine the reader is presented with a some- 

 what detailed account of the distribution of snow and ice in the North 

 American Cordilleras, and on the western side of that continent generally ; 

 and there is nothing to add here in reference to the occurrence of similar 

 phenomena on the Atlantic slope, or in the extensive regions to the north- 

 east of the Great Lakes. It is not until we pass beyond the Polar Circle, 

 and reach the island groups which lie to the north of the continental mass 

 of land proper, that there can be any question as to snow remaining per- 

 manently upon the surface, or forming itself into glaciers. As has been 

 already shown, however, by lar the larger part of these islands are not only 

 not the scene of any general glaciation, but they are, even on high ground, 

 to a very considerable extent bare of snow during the summer months. 



Enough has been said, as it appears to the writer, in regard to the present 

 distribution of snow and ice throughout the world, to enable the reader to 

 have a fairh' complete idea of the climatic conditions by which this distribu- 

 tion is governed. All that remains to be done, before passing to the next 

 division of our inquiry, is to call attention to the bearing of the facts which 

 have been presented in this section on certain points which are of special 

 importance in (connection with the problem before ns. There are, however, 

 other points which will not be touched upon at this stage of our discussion 

 because they can be brought forward in a more effective manner in a 

 succeeding section, after some of the phenomena of the glaciation of a past 

 epoch have been laid before the reader. These will be presented very suc- 

 cinctl}^ with such comments as may seem desirable for the proper appre- 

 ciation of their bearing. 



The present discussion may be opened by a few words in regard to the 

 general subject of precipitation, in addition to that which has already been 

 given in the preceding chapter. It is especially desirable that a clear idea 

 should be had of the climatic conditions requisite in order that snow may 

 accumulate in any particular i-egion in large quantity, and of the manner in 

 which this superabundant supply is converted into ice. 



It is well understood that precipitation takes the form of snow when it 

 falls through or comes in contact with air chilled below the freezing-point. 

 It would appear that to many Avho have written extensively on glacial 



tographs of ordinary icebergs, taken in lower northern latitudes, for any showing this distinctly stratified appear- 

 ance. Stratification is indeed visible in some; but of an entirely different character from tliat of tlie floebei-g, it 

 being finer and curvilinear ; just such, in fact, as would be expected to occur in fragments of large glaciers. 



