300 Proceedings of the Royal Physical Society. 
of origin which receives further support from their uniformity 
of lithological character over large areas, and the fineness 
and evenness of their lamination. Their fossils, again, point 
to much the same conclusion. 
We certainly have no reliable data on which to found an 
estimate of the time required for the formation of this 
enormous accumulation of marine deposits. The grey- 
wackes may have been formed at the rate of 1 foot in 
2000 years; which, as their estimated thickness is 8000 
feet, gives us 16,000,000 years for the whole of this section. 
The more argillaceous portion, which may be set down as 
about one-third of the entire thickness, 7.¢., 4000 feet, must 
have been formed at a slower rate. If we set this rate at 
probably 1 foot in 3000 years, we shall be within the mark. 
This gives us an additional 12,000,000 years; making a 
total of 28,000,000 years for the Middle and Upper 
Salopians. 
As for the time required for the accumulation of the 
deep-water graptolitiferous beds and the deep-sea clays which 
now form the Pale Slates, we can only judge by analogy. 
If the rate of paleontological change may be taken as a 
guide, then these two subdivisions of the Lower Salopian 
must have taken longer to form than the whole of the 
overlying 12,000 feet of greywackes and argillites. This 
view will, I am sure, commend itself to all paleontologists, 
and to the majority of the geologists who have studied 
these rocks in the field. I shall therefore add another 
28,000,000 years for the formation of the rocks in ques- 
tion. 
Uneonformity at the Base of the Salopian Rocks in Shrop- 
shire and clsewhere—Near Moffat, and at one or two other 
localities in Scotland, there is no clear evidence of any 
great physical break between the Salopian Rocks and the 
Ordovicians — probably because deep-sea conditions per- 
sisted there throughout the whole period from Arenig times 
to near the end of the Lower Salopian. In Ayrshire the 
break between the two is more marked. In Craven, as 
Professor Hughes showed many years ago, a well-marked 
unconformity exists—one to which I am myself disposed to 
