258 DARWIN ON PARALLEL ROADS. CHAP. xm. 



One is so nearly dry that it can be walked over at low water, 

 and another, no longer covered by the sea, is supposed to 

 have recently dried up in consequence of a small alteration 

 in the relative level of sea and land. ( Similar straits,' 

 observes Mr. Chambers, 'hovering, in character, between 

 sea and land, and which may be called fords, are met with 

 in the Hebrides. Such, for example, is the passage dividing 

 the islands of Lewis and Harris, and that between North 

 Uist and Benbecula, both of which would undoubtedly appear 

 as cols, coinciding with a terrace or raised beach, all round 

 the islands if the sea were to subside.'* 



The first of the difficulties above alluded to, namely, the 

 non-extension of the shelves over certain parts of the glens, 

 might be explained, said Mr. Darwin, by supposing in 

 certain places a quick growth of green turf on a good soil, 

 which prevented the rain from washing away any loose 

 materials lying on the surface. But wherever the soil was 

 barren, and where green sward took long to form, there may 

 have been time for the removal of the gravel. In one case 

 an intermediate shelf appears for a short distance (three 

 quarters of a mile) on the face of the mountain called Tomb- 

 hran, between the two upper shelves, and is seen nowhere 

 else. It occurs where there was the longest space of open 

 water, and where the waves may have acquired a more than 

 ordinary power to heap up detritus. 



The unequal number of the shelves in valleys communi 

 cating with each other, and in which the boundary rocks are 

 similar in composition, and the general absence of any shelves 

 at corresponding altitudes in glens on the opposite watershed, 

 like that of the Spey, and in valleys where the waters flow 

 eastward, are difficulties attending the marine theory which 

 have never yet been got over. Mr. T. F. Jamieson, before 



* Ancient Sea Margins, p. 114, by R. Chambers. 



