302 Proceedings of the Royal Physical Society. 
Rocks has a much wider extension than appears hitherto 
to have been recognised. 
Returning to the Longmynd unconformities: we find the 
Cambrian Rocks themselves, in their turn, overstepping the 
edges of the old Pre-Cambrian Rocks of the Longmynd, and 
they are known to lie even upon the edges of a series 
of eruptive rocks—the Uriconians—which many competent 
geologists regard as unconformable below the Longmyndians 
themselves. 
Ignoring these older unconformities, therefore, we need at 
this point only take into account the thickness of strata 
which there is reason to believe was removed by denudation 
after the close of the Ordovician Period, and prior to the 
deposition of the Lower Salopian Rocks. Taking this amount 
at 4000 feet, as shown by the Survey sections, this, at the 
average rate of denudation here employed, gives us an 
additional 12,000,000 years. 
Time required for the Formation of the Upper Ordovician 
Rocks.—Confining our attention for the present to the 
Upper Ordovician Rocks of the Longmynd area, we find, 
from a measurement of the Geological Survey “ Horizontal” 
Sections, that these rocks are fully 4000 feet in thickness, 
and consist mainly of marine sediments, together with some 
volcanic rocks. The general character of the series as a 
whole is suggestive of quiet deposition, while the changes in 
the faunas as the rocks are traced from below upward, 
point unmistakably to a great length of time. If we assume 
that, as a whole, this series was formed at no greater rate 
than that of 1 foot in 3000 years (which in this case, as 
in others of the same nature, seems to me to be a rate far too 
rapid), 4000 feet would require 12,000,000 years. 
Unconformity at the Base of the Bala Series—Considering 
that the Longmynd district has been repeatedly the centre 
of axes of upheaval, and considering, further, that volcanic 
action was rife throughout the older period there, it seems 
to me advisable to regard the unconformity which brings the 
Bala Rocks into direct contact with the Tremadoc strata on 
the east side of Caer Caradoc, as probably due to contempor- 
aneous disturbances and upheavals of the same nature as 
