258 DARWIN ON PARALLEL ROADS. CHAP. xiiL 



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 laud, 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 ap- 

 pear 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 bai*ren, and 

 where greensward took long to form, there may have been 

 time for the removal of the gravel. In one case an interme- 

 diate shelf appears for a short distance (three-quarters of a 

 mile) on the face of the mountain called Tombhran, between 

 the two uj)per 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 Spej^, and in vallej^s where the waters flow 

 eastward, are difficulties attending the marine theory Avhich 

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



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



