Trof. Milne- — Across Europe and Asia. 63 



Early next morning we entered the Nankan Pass, and after four 

 hours' scrambling and sliding down over the bed of a frozen torrent, 

 which was bounded by bare rugged granitic mountains, we reached 

 the lower end of the defile, which, I think, presents the finest and most 

 romantic scenery between Pekin and St. Petersburg. At the lower 

 end of this pass, after passing the famous gateway of Kiiyungkwan, 

 you see the junction of a limestone rock with the granite, the latter 

 being overlain by the former. This limestone, together with that 

 which I have before mentioned, is supposed to underlie the Coal 

 formation, and to be of Devonian age — a view which is confirmed by 

 the Brachiopoda which are said to occur in it. 



The country at the lower end of the pass I found to be very 

 bouldery, being covered with blocks of limestone and granite, which 

 had rolled down from the mountains. These gradually became less 

 and less, until we were at last upon a cultivated flat dusty plain, 

 over which we travelled until we reached Pekin on the 14th. 



I have not said more about my journey from Kalgan to Pekin, 

 because this portion has been already described by Mr. Pumpelly in 

 the Smithsonian Contributions to Knowledge, No. 202, in considerable 

 detail, to which I have but little if anything to add. 



When looking at high rugged mountains like those I had just passed 

 through in the Nankan Pass, and like others which I had seen in other 

 parts of the world, as, for instance, in N.W. Arabia, it often occurred 

 to me that these mountains must be wearing away at an enormously 

 greater rate than others do where the inclination is not so steep. 

 Mechanical degradation, to which such moimtains are subject, might 

 be considered as roughly measurable by the action of gravity 

 pulling particles to lower levels. A particle lying on a level plane 

 is not affected by this action, and there is no degradation. But 

 particles lying on inclined planes are pulled downwards, and the 

 steeper the plane the greater is the pull, and consequently the more 

 effective and rapid is the action. Thus, for instance, if we see a rock 

 balanced on the side of a steep mountain, we know that there is 

 more danger of its rolling down, and that when it does roll, it will 

 do so more rapidly than if it had been placed upon the side of a 

 hill which was comparatively less steep. This renders it danger- 

 ous to travel beneath a steep mountain, or through a deep cutting, 

 and this is one way of looking at the recognized fact that steep 

 mountains are degraded at a greater rate than those with slopes 

 more gentle. 



In order to gain some conception of what these rates are in com- 

 parison with the slopes upon which they occur, we must consider 

 the conditions tending to accelerate or retard the downward pull of 

 gravity. Amongst the many points to be considered when com- 

 paring the degradation or falling down of material upon two slopes, 

 the following appear to be of importance : 



1st. The force tending to loosen any material lying upon tlie 



surface of a mountain. 

 2nd. The circumstances under which material may obtain its 

 initial motion. 



