114 P. Macnair Sf J. Reid—On the Old Red of Scot /and. 



spntences previous to this, he says the} r represent the general 

 submarine level to which the Highland region was reduced. Once 

 more, as before quoted, he does not seem to be very sui'e as to 

 whether this base-level of erosion was the action of a group of 

 lakes or of the sea. In trying to fix the geological date of this 

 vast denudation, he says that it must have been accomplished 

 between the time of the Lower Silurian and that of the Lower Old 

 Red Sandstone system. Probably the waste begun in the Lower 

 Silurian period and continued during the vast series of ages that 

 extended into the period of the Old Red Sandstone. Here again, 

 we would think he was referring this immense denudation to the 

 group of fresh-water lakes, as he refers to it as being continued 

 during the vast series of ages that extended into the period of 

 the Old Red Sandstone. 



We must confess to having a great difficulty in following Sir 

 A. Geikie's description of the succession of physical events that 

 took place immediately after the plication and upheaval of the old 

 Palaeozoic mountain chain. But we think the following may be 

 taken as a fair representation of his meaning — That after the 

 plication and metamorphism of the crystalline rocks of the High- 

 lands, the whole mass was reduced to a plain of marine denudation 

 by a sea which has left no traces whatever of this immense denuda- 

 tion in the shape of later deposits, and that afterwards this plain of 

 marine denudation was depressed into regions occupied by fresh- 

 water lakes which, creeping backwaixls, threw down the deposits 

 now known as the Lower Old Red Sandstone formation. Generally 

 speaking, the whole tenour of the chapter dealing with the tableland 

 of the Highlands seems to indicate that an extensive marine denuda- 

 tion reduced the massive mountain chain of the Highlands to a base- 

 level of erosion altogether distinct from the later denudation which 

 threw down the massive conglomerates of the Old Red Sandstones 

 with its 20.000 feet of sandstones and shales. 



Now we would maintain that this first marine denudation, which 

 levelled down the old Palaeozoic mountain chain, was identical with 

 the deposition of the Upper Silurian and Lower Old Red Sandstone 

 rocks, pointing out that, though Sir Archibald Geikie believes that 

 it was the sea which levelled down the old continent, he cannot 

 show us anywhere the remains of this immense denudation, all the 

 later rocks lying upon it being of Old Red Sandstone age, and 

 consequently having a fresh-water origin. Further, we cannot see 

 how, if the whole land surface were reduced to a level plain 

 first by marine denudation, this plain could be now preserved, 

 seeing that subsequently it was ridged up into isolated basins to 

 hold the fresh-water lakes. It is evident, then, that Ramsay, Geikie, 

 aud their followers distinctly recognize two periods of denudation 

 after the plication of the Lower Silurian rocks into a mountain 

 chain. The first period of denudation cut down this mountain 

 chain to a dead level of erosion, called by them a plain of marine 

 denudation, which was produced by the action of the sea. This 

 period, however, strange to say, seems to have left no traces behind 





