298 Proceedings of the Royal Physical Society. 
disposed to assign a comparatively small chronological value 
to the Old Red unconformities, as they appear to be due to 
undulations of much smaller wave-length, greater amplitude, 
and higher frequency, than we have reason to believe pre- 
vailed during many other periods. The nearest parallel case 
in progress at the present day seems to me to be the zone 
of country which embraces the Nile Valley and the borders 
of the Red Sea, where, apparently, terrestrial undulations of 
the same nature are throwing the earth’s surface into close 
alternations of upland area and area of depression. 
According to the view here taken, the time required for 
the formation of the Lanarkian Rocks, and their probable 
chronological equivalents the Glengariff Grits, is counted 
partly with that occupied by the formation of the Ludlow 
Rocks, partly with that of the lowermost part of the 
Caledonian Old Red; while to the time represented by the 
formation of the Caledonian Old Red (20,000 feet or more) 
will probably have to be added that represented by at least 
the upper half of the Orcadian Rocks, of the full thickness 
of which perhaps not more than two-thirds at present 
remain. z 
The true measure of time represented by the rocks at 
present referred to could be estimated probably with the 
nearest possible approximation to accuracy, if we knew the 
maximum thickness of marine limestone formed in the 
interval between the close of the Ludlow Period and the 
commencement of the Carboniferous. All we can say upon 
this point is that in the Continent at least 20,000 feet of 
marine strata of various kinds are referred to this horizon, 
and of these at least 5000 feet consist of marine limestone. 
There does not appear yet to be any precise figures in 
relation to this; but assuming that only 5000 feet of marine 
limestone were formed during the period when so many 
physical and biological changes went on in other parts, 
that alone gives us for the Devonian Period a time interval 
of 125,000,000 years. It may be remarked here that, where 
the Devonian Limestone attains this exceptional thickness, 
no unconformity can be traced between the Silurian Rocks 
and the Carboniferous. I have therefore taken no account 
