Vice-President’s Address. 301 
attach much greater importance even than he did. In 
South Wales the unconformity between the Salopian and 
Ordovician Rocks is very pronounced. But it is in Shrop- 
shire, and especially in the Longmynd area, that the evidence 
of a break of enormous extent is most clearly to be seen. 
There the May-Hill, Llandovery, or Lower Salopian, beds are 
seen to overstep the edges of every member of the Ordovician 
Rocks, themselves 15,000 feet in thickness—the whole of 
the Cambrians, 3000 feet or more—and, finally, they over- 
step the edges of the older Longmyndian Rocks, whose 
thickness cannot be less than 8000 feet, and may well be 
between twice and three times that thickness. Taking the 
lowest estimate, this gives us for the total thickness of 
rock overstepped by the Salopians in the Longmynd area 
26,000 feet (or possibly half as much more than that). 
There can be no question here of overlap to any appreci- 
able extent, as the Pentamerus Beds are present over nearly 
the whole area, which shows that we are not dealing with 
a case either of deposition of marine strata on the flanks of 
an old island, or of deposition in an area of very unequal 
subsidence. Nothing can be clearer than the evidence that 
the whole of the unconformity is pre-Salopian. But, on 
the other hand, there is abundant internal evidence to show 
that the whole of this vast hiatus cannot be referred to 
denudation following the close of the Ordovician Period. On 
the eastern side of the Longmynd area, at Caer Caradoc, the 
Bala Rocks repose directly upon rocks of Tremadoc age— 
the intermediate rocks of Lower Ordovician age, fully 10,000 
feet in thickness within a short distance of this spot, being 
here entirely absent. Mr Watts informs me in a letter that 
in places the Salopian Rocks lie even upon the rocks of 
Longmyndian Age. Part, therefore, of the enormous gap 
occurring at the base of the Salopian in the Longmynd 
area is referable to an unconformity of pre-Bala age. 
I have mentioned above that the equally vast uncon- 
formity at the base of the Upper Old Red in the English 
Lake District is referable, in like manner, to the cumulative 
effect of several unconformities of widely different ages. 
The unconformity at the base of the Upper Ordovician 
