Vice-President’s Address. 295 
consists of three zones—a Lower, graduating down into the 
Upper Silurian shales; a Middle, bounded both above and 
below by an unconformable junction; and an Upper, which 
shades up into the Carboniferous Rocks.” This relationship 
is most clearly and accurately shown upon the “ Horizontal ” 
Section of the “Geological Survey of Scotland,” Sheet 3, 
of which Sir Roderick Murchison gives a condensed diagram- 
section in “ Siluria,” 4th ed., p. 249. 
Chronological Value of the Old Red Unconformities.—It 
appears to me that the complicated relationships existing 
between these several groups of strata do not warrant us in 
assuming, in this case at any rate, that we should be right in 
adding on to the time required for the actual formation of 
each subdivision of the Old Red other intervals of time which 
the unconformities would seem to imply. We are quite 
justified in taking it for granted that Sir Archibald Geikie’s 
suggestions upon this point offer us a satisfactory explanation 
of some of the chief difficulties. According to this view, 
deposition and upheaval went on simultaneously in closely 
adjacent areas, so that older rocks were folded upward, and 
underwent denudation along one zone of the surface; while 
along a parallel zone, not many miles away, a downward 
phase of the terrestrial undulations gave rise to the condi- 
tions suitable for deposition. I have endeavoured to show 
elsewhere’ that this was probably also the case in later 
Ordovician times in the north-west of England, where at one 
part there appears to be a perfectly unbroken succession from 
Lower Ordovician into Upper, while along another zone rocks 
of Bala age were deposited upon the upturned edges of the 
Lower Skidda Slates (? Upper Cambrian). 
In the case of the unconformities connected with the 
various Old Reds, it appears to me that the facts warrant 
us in going further than this, and in assuming that the 
undulations to which Sir Archibald refers were not simple 
up-and-down movements confined to particular localities, 
but that they actually progressed in course of ages from 
one part to another. Early in the Silurian Period, according 
1“ On the Stratigraphical Relations of the Skidda Slates,” Proc. Geol. 
Assoc., vol. ix., No. 7. 
