12 Proceedings. 
The Tremadoc Slates—as their name indicates—are so called 
from the town of Tremadoc in North Wales, where they are 
well exposed, and where they were first observed in 1846 by 
Prof. Sedgwick to be distinct lithologically from the beds above 
and below them. ‘Ten years later Mr. J. W. Salter determined, 
after examination of the fossils from the Slates in this district, 
that they must rank as a sub-formation. 
Before describing in detail the three areas in Wales, and the 
one in Shropshire, where we find these rocks, we will consider 
first their position in the stratigraphical succession. For this 
purpose I must direct your attention to the Geological Record. 
From it we see that the Tremadoe Slates uccur low down in 
the Primary or oldest division. English geologists, or, rather, 
I should say the Geological Survey, class the beds with the 
Cambrian or lowest system ; foreign geologists, on the other 
hand, place them at the base of the Ordovician ; the reason for 
this we shall consider later on, when dealing with the fauna 
of the formation. 
The geologist has four main guides to help him in forming a 
chronological classification of the rocks of the earth’s crust :— 
(1) by their visible superposition ; 
2) ee distinctive lithological character ; 
(3) 4, containing fragments of rocks derived from 
other formations, the actual place of which is known ; 
(4) by their distinctive fossils. 
We will consider briefly the relative value of these guides in 
determining the age of the Primary rocks of Wales. 
(1) The law of superposition (that is to say, that the strata 
which lie on the top are younger than those they rest upon). 
This law is not applicable among the Welsh hills, where the 
strata are almost invariably bent and contorted, where masses 
of rock may have been brought up from below and pushed on 
to the top of rocks younger than themselves. 
(2) The lithological character of a rock is helpful in dis- 
tinguishing one series from another, but alone it can give 
no definite evidence of the age of a series; nevertheless an 
