Niles.] 326 [March 6, 



tation, while the " Sub-alpine Zone," or " Kegion of Coniferous 

 Trees," occupies the lower slopes. Let us now notice the features of 

 the corresponding physiographic zones. 



ZOXE OF WEATHERING. 



This zone, among the Alps, corresponds most nearly with the 

 " snow-region," although, since the marked recession of the snow-fields 

 and glaciers, it extends in many places considerably below the limit of 

 perpetual snow. It is in this region that the greatest vicissitudes of 

 climate and the most sudden and violent changes of temperature 

 occur. The alternation between heat and cold, which here takes 

 place in clear weather between each day and night, gives the atmos- 

 phere great mechanical power over the rocks of the exposed ridges 

 and steep slopes, from which the snow has been removed by winds 

 and avalanches. The expansions and contractions of the rocks, 

 which take place rapidly, tend not only to disintegrate the surfaces, 

 but also to throw off irregular fragments. The almost daily melting 

 and freezing of the water in seams and fissures is a powerful mechan- 

 ical agent for breaking and loosening larger masses of rock. When 

 the rocks are of that character which permits them to be more easily 

 broken than disintegrated by these meteoric changes, as is the case 

 with many crystalline rocks, then the unprotected slopes and ridges 

 of this region acquire those rough and angular features which charac- 

 terize mechanically broken rock. As the surface features of the un- 

 covered rocks have been produced chiefly by those mechanical actions 

 of the atmosphere usually called weathering, I speak of this region of 

 the mountains as physiographic ally the Zone of Weathering. The 

 principal ridges and great peaks of this zone are the ones most ex- 

 posed, and they exhibit the wildest forms of rock to be found among 

 the mountains. In the nearer views of this region, steep declivities 

 with craggy outlines and angular contours predominate over the other 

 forms of rock-surface . In distant views the profiles are steep, broken, 

 irregular, or serrated, and stand out in bold contrast with the curved 

 and gentler slopes below. A fine exhibition of this contrast is seen 

 upon the northern face of the Chain of Mont Blanc, looking from the 

 Brevent. 



I do not include, in this description, the surface forms of the snow- 

 fields of this zone, because they obscure the features which the 

 mountains would otherwise present, and also protect them from the 

 action of the atmosphere. 



