238 puocEEDiNGS 0^ THE GEOLOGICAL SOCIETY. [March 6, 



the east. Then, as now, a strong tidal current from the west — aided 

 also, we may believe, by the Gulf-stream — would set into these 

 channels, and tend to raise the water on the west side of the islands 

 higher than on the east. This is the case at present, when the 

 mean level of the sea is some four feet higher at the west than at 

 the east end of the Caledonian Canal. The amount of elevation, 

 however, would be modified as the currents could flow with more 

 or less freedom through the open straits to the east ; and were these 

 outlets whoUy closed, the water, no longer able to flow away to the 

 east, would accumulate on the other side of the barrier, and a con- 

 siderable rise in the sea-level in the western bays would necessarily 

 be established. 



Let us now look at what would happen during the slow, regular 

 rise of the land. So long as the channels were open throughout, as 

 the land rose the sea would fall on the shore, lower and lower, we 

 may say, at each successive tide, and spread the detritus in a 

 uniform layer over the declivities. When, however, the bottom of 

 one of the straits rose above the surface at low water, a new state 

 of things would commence. The tidal currents would be checked 

 in their eastward flow, and the sea-level in the western bay would 

 begin to rise, and continue rising till the strait was wholly closed 

 even at high tide. During the intermediate period, though the land 

 was rising, the western sea was not falling, but was virtually 

 stationary. When at length the rise in the land entirely shut the 

 strait, then (the full efiect of the interposed barrier being attained) 

 the sea in the western bays would anew begin to fall uniformly and 

 regularly as the land rose. We have thus an interval more or less 

 prolonged of stationary sea-level interposed between two periods of 

 regular uniform sea-depression. During this interval the sea would 

 act for a long time continuously on one line of land, and thus make 

 a deeper indentation into the detritus. This indentation is one of 

 the present roads ; it marks the period during which the col rose 

 from the level of low to that of high water, and consequently cor- 

 responds with it in elevation. The notch in the land, the Koad, thus 

 represents the transition period when the strait was partly open, 

 partly closed during each tide, and the pause, as it were, in the fall 

 of the sea-level thus occasioned. Once the barrier was fully esta- 

 blished and the strait permanently shut, the regular continuous fall 

 of the water, and the consequent uniform unbroken slope of the 

 declivities would recommence, and continue till a second col was 

 reached, when a second line would form. 



The formation of lines of erosion or roads at the level of these cols 

 seems thus the necessary result of the damming up of the western 

 tidal currents during the gradual rise of the land. No one can 

 doubt that the rise in the sea-level thus occasioned would be very 

 considerable, who has observed the strong currents that run among 

 the western isles, or in the wider channel between Cantyre and 

 Ireland to the south of this tract of coast, and round Cape Wrath 

 and through the Pentland Firth on the north. Were the reefs in 

 the latter, marked by the Merry Men of Mey and other too well- 



