CHAP. xni. DARWIN ON PARALLEL ROADS. 257 



opinion that the shelves "were formed when the glens -were 

 still arms of the sea, and, consequently, that there never were 

 any seaward barriers. According to him, the land emerged 

 during a slow and uniform upward movement, like that now 

 experienced throughout a large part of Sweden and Finland; 

 but there were certain pauses in the upheaving process, at 

 which times the waters of the sea remained stationary for so 

 many centuries as to allow of the accumulation of an extra- 

 ordinary quantity of detrital matter, and the excavation, at 

 many points immediately above the sea-level, of deep notches 

 and bare cliffs in the hard and solid rock. 



This theory I adopted in 1841 ("Elements," 2d ed.), as ap- 

 pearing to me less objectionable than any other then proposed. 

 The phenomena most difficult to reconcile with it are, first, the 

 abrupt cessation of the roads at certain points in the different 

 glens; secondly, their unequal number in different valleys 

 connecting with each other, there being three, for example, in 

 Glen Eoy, and only one in Glen Spean ; thirdly, the precise 

 horizontality of level maintained by the same shelf over a space 

 many leagues in length, requiring us to assume that during 

 a rise of 1156 feet no one portion of the land was raised eveu 

 a few yards above another; fourthly, the coincidence of level 

 already alluded to of each shelf with a col, or the point form- 

 ing the head of two glens, from which the rain-waters flow 

 in opposite directions. This last-mentioned feature in the 

 physical geography of Lochaber Mr. Darwin endeavored to 

 explain in the following manner. He called these cols 

 *' land-straits," and, regarding them as having been anciently 

 sounds or channels between islands, he pointed out that 

 there is a tendency in such sounds to be silted up, and 

 always the more so in proportion to their narrowness. In a 

 chart of the Falkland Islands, by Capt. Sullivan, E.N., it 

 appears that there are several examples there of straits where 

 the soundings diminish regularly towards the narrowest part. 



