Vice-President’s Address. 305 
The Durness Limestone.—It happens in the case of the rocks 
of Middle and Upper Cambrian age that we are able to a 
certain extent to check the computation regarding their age 
by figures obtained from another source. In the north-west 
of Scotland the rocks containing Olenellus and a Lower 
Cambrian fauna (which correspond in time with the upper 
part of the Harlech Rocks) are overlaid by a thick mass of 
marine limestone—the Durness Limestone—whose higher 
beds yield fossils with a Tremadoc facies. This limestone 
would therefore seem to be the equivalent in time of all the 
North Wales strata from the base of the Menevian to the 
top of the Tremadocs. Mr Peach tells me it may be from 
1500 to 2000 feet in thickness or even more, as its top is not 
known for certain. Relying upon these lower estimates, it 
would seem that the 7350 feet of argillaceous beds in North 
Wales are chronologically equivalent to 1500 feet of lime- 
stone in the north-west of Scotland, which is in the 
proportion of about four and a half of fine marine mud to 
one of limestone —a proportion which seems to hold good in 
other cases than this. If we take the time requisite for the 
formation of the Durness Limestone at 1 foot in 25,000 
years, this gives us 37,500,000 years, which is an estimate 
not very different from that arrived at in connection with 
the Cambrian Rocks of North Wales. 
Adding to this estimate the time required for the formation 
of the Lower Cambrian Rocks of Wales, we get for the whole 
Cambrian Period 42,000,000 years. 
Summary regarding the Age of the Lower Cambrian 
Locks ;— 
Time in Years. 
From the Commencement of the Tertiary Period to the 
Present Day, . 93,420,000 
Time represented by the Farina of ne Chalk, Tianee 
Greensand, Gault, and Lower Greensand, . : : . 31,400,000 
Pre-Cretaceous Unconformity in Britain, . : 13, 200,000 
Time required for the Deposition of the Jurassic- Wealden Bees, 59,400,000 
Time represented by the Tyrolian and Panormian Rocks, . . 87,500,000 
Unconformity at the Base of the Lower New Red in Britain, . 45,000,000 
Duration of the Upper Carboniferous Period in Britain, . . 81,815,000 
Carry forward, . 361,735,000 
