304 Proceedings of the Royal Physical Society. 
the Cambrian Rocks, properly so-called, are certainly not 
less than 13,000 feet in thickness) The Harlech and 
Llanberis Rocks, measured up to the base of the Menevians, 
are, according to my calculation on the ground, a little over 
6000 feet. To this we have to add 200 feet for the Mene- 
vians, 5150 feet for the Lingula Flags, and 2000 feet for the 
Tremadoc Rocks. This gives a total thickness for the Cam- 
brians of 13,350 feet, which is, if anything, below, rather 
than above, the mark. The Harlech beds are largely 
quartzitic, and may have been formed at a somewhat more 
rapid rate than that generally taken into computation here, 
On the other hand, the remaining strata consist largely of 
what must originally have been fine marine loam, mud, and 
clay, which were slowly and quietly deposited at some 
distance from the land. In dealing with these Cambrian 
Rocks, I should be disposed to modify the figures used 
generally in the foregoing computation, and to allow a 
somewhat higher rate for the formation of the Harlech 
and Barmouth Rocks, seeing that these consist largely of 
a coarse greywacke, If we set their rate of accumulation 
at 1 foot in 1000 years, this would probably be a fair 
average, seeing that these rocks include argillaceous bands 
in the proportion of about one-fifth of the whole. Taking 
their collective thickness at 6000 feet, this gives us 6,000,000 
years. 
The overlying Menevian strata contain a widespread, if 
thin, deposit of manganese ore (which, by the way,is a true bed, 
and not a vein), whose general aspect lends additional con- 
firmation to the view suggested by its fossils that in this case 
we are dealing with a deep-sea deposit. It has already been 
stated that the petrographical nature of both the succeeding 
Lingula Flags and Tremadoc Rocks is also suggestive of quiet 
and slow deposition—a view equally well borne out by the 
fossils which occur in these rocks. I was so much impressed 
by these features when studying these rocks in North Wales 
a few years ago, that I should feel quite justified in setting 
their rate of formation at 1 foot in 5000 years. Their 
ageregate thickness in North Wales is 7350 feet, which, at 
the rate suggested, gives us a period of 36,000,000 years. 
