Obserfations on the Silurian Rocks 
of Porth Wales. 
BY T. McKENNY HUGHES, M.A., FRS., 
WoopwarDIAN PRorFEssor OF GEOLOGY, CAMBRIDGE. 
Read January 8th, 1886. 
[* the following Paper I propose to offer to the Society 
(1) a description of some Sections across the Silurian Rocks 
of our district, (2) further observations on districts which I have 
already noticed in communications to the Society, and (3) a 
tentative correlation of the various horizons indicated with 
those of other adjoining areas, such as may at any rate 
suggest lines of enquiry to those interested in palzontological 
stratigraphy. I have not, however, observed the above order, 
believing that in communications of this kind it is generally 
better to reverse the order and to state early the results arrived 
at, and then discuss the facts and observations upon which 
the conclusions have been formed. 
I shall therefore first give in columns five typical sections ; 
next, comments upon them; and then I shall describe the 
succession in our district, pointing out what horizon or zone in 
the northern or southern series each subdivision, from its 
fossils or stratigraphical position, seems to represent. (See 
table on following page.) 
In this table it will be seen that the five sections referred to 
readily fall into two groups, viz., the southern, of which the 
types are found in the Counties of Carmarthen and Hereford, 
and the northern are represented in Denbigh, Westmorland, and 
Yorkshire. I have not carried the comparison further than 
what may be called the adjoining areas, but I have not strictly 
confined my references to the limits of the Counties named. 
There is across central England and Wales an axis of more 
marked movement, resulting in barriers which have affected the 
stratigraphical structure throughout almost the whole geological 
series. It separated the northern volcanic region of the Upper 
Cambrian, or, as it is the fashion now to call it, the Ordovician 
from their southern equivalents, in which traces of volcanic 
activity of that age are rare. “ It is very marked in the Silurian 
Rocks with which we are now more especially concerned. 
B2 
