Correspondence — Messrs. JIacnair and Eeid. 571 



but fair to consider he bad been led by Sir A. Geikie's description 

 of the succession of physical events in Scotland after the plication 

 of the earlier Palaeozoic rocks, to infer that there had been two 

 distinct periods of denudation, the one marine and the other fresh- 

 water. We sincerely trust that this explanation will be acceptable 

 to Professor Davis of the reading we placed upon his footnote. 



But let us now turn to the much more important subject : Is there 

 any evidence of two distinct periods of denudation — a marine, to 

 which the plain of marine denudation is to be referred, and a 

 fresh- water, to which the Old Red Sandstone deposits are to be 

 aligned ? Professor Davis, at least on this point, evidently believes 

 that there must have been a vast denudation prior to the deposition 

 of the Old Red Sandstone deposits, and that this denudation may 

 have taken place without leaving any trace of its deposits. But he 

 dues not decide as to whether this denudation was subaerial or 

 marine. If he believes that a vast subaerial denudation took place 

 partly previous to, and partly contemporaneous with, the deposition 

 of the Old Red Sandstone rocks of Scotland, then we are at one 

 with him. But if, on the other hand, he believes with Sir A. 

 Geikie, that a great marine denudation took place which has left 

 not a "rack behind" in the shape of deposits, but is only repre- 

 sented by a plain of marine denudation, then we beg to express 

 our dissent. 



It seems to us Professor Davis takes but a partial and limited 

 view of the structure and relationship of the Scottish Old Red 

 Sandstone to the older Palaeozoic mountain-chain. He states that 

 the Old Red Sandstone lies unconformably upon the floor of ancient 

 rocks. This is doubtless correct so far as the Moray Firth area, and 

 all Old Red Sandstone areas north of the Grampians, are concerned ; 

 but had he extended his traverse of the Old Red Sandstone to the 

 boundary fault between Ayrshire and Midlothian he would hive 

 found that the Old Red Sandstone conformably succeeds the Silurian 

 of these districts. It is here, in our view, where the missing link is 

 found. With Ramsay, Geikie, and others, Professor Davis seems to 

 overlook the significance of this important fact. In this connection 

 a feature of some interest may be referred to. In Lanarkshire 

 a band of shale about 5,000 feet above the base of the Old Red 

 Sandstone contains Orihoceras dimidiatum, Spirorbis Lewisii, and 

 a graptolite. These being unquestionably Silurian fossils, may not 

 this band of shale and underlying sandstones, conglomerates, etc., 

 be the equivalents of the Downton Sandstones ? Again, we have 

 shown that the Caithness rocks must be assigned to a higher horizon 

 than those of Strathmore, while we think we have given strong 

 evidence, both physical and palaeontological, for believing that the 

 whole of the series from the passage beds of Lanarkshire, through 

 the middle series of Strathmore, into the highest members of the 

 group in the Orkney Islands, are of marine origin ; and it is to this 

 age we would refer the plain of marine denudation seen in the 

 Highland mountains. 



Professor Davis advances two objections to the plain now seen 



