518 Geological Society: Anniversary Address, 1843. 
beneath, they might hereafter be found to contain other and distinet 
forms of animal life. 
The inquiry of Sir H. De la Beche has dispelled our ignorance. 
Instructing a number of intelligent young surveyors how to apply 
trigonometrical mensuration to stratified rocks, and patiently follow- 
ing up each mineral mass through its change of conditions upon its 
strike and throughout every contortion, the Director of the Survey 
has now clearly ascertained that the rocks to the north and west 
of the Towey in Caermarthenshire, as well as those to the north of 
Haverfordwest in Pembrokeshire*, instead of being an undefined 
assemblage, to which the term Cambrian had been applied, are in 
truth nothing but the very same Lower Silurian rocks which had 
been pointed out on the east and south, and which (the Llandeilo 
flags being much more important than the Caradoc sandstone) are 
repeated in great folds and undulations to the north and west. Often 
parting with their calcareous matter, these strata, often assuming 
a crystalline slaty cleavage, are in some tracts highly altered by 
the intermixture of trappean rocks, both of contemporaneous 
origin and subsequent intrusion. But in these altered rocks the 
Ordnance surveyors have detected true Lower Silurian fossils, and 
have thus, by zoological evidence as well as by geometrical admea- 
surements, convinced themselves that the rocks so very different in 
aspect, are nothing more than repetitions of the same fossiliferous 
strata which have been described upon the south and east. Such 
results, obtained amid strata so obscured by change, is one of the 
very highest triumphs of geological field-work ; and I therefore wish 
to be foremost in recognising the deserts of the labourers who have 
obtained them, among whom the Director particularly cites Mr. 
Ramsay, already so favourably known to us by his geological map 
and model of the Isle of Arran. 
In looking at the Ordnance Maps of North Pembroke, which have 
recently been coloured, and will shortly be issued to the publie, it is 
surprising to see how symmetrical order has been obtained out of 
such a labyrinth, and how the fragments and pieces of such a 
patch-work are brought together. I have the authority of Sir 
H. De la Beche to state, that in some districts the convolutions 
are so rapid as to reproduce the same band of contemporameous 
trap, in perfectly parallel lines, no less than ten or twelve times in 
the width of a mile, whilst bosses of eruptive trap are so numerous 
as to defy analytical research. Although I only passed quickly 
over the tract of North Pembroke, and ought therefore, never to 
have added it to those portions of my work which were more care- 
fully executed, I have still sufficient recollection of it to admire 
the beauty of the new delineations. If I may be allowed to sug- 
gest a parallel between it and districts which I have more mi- 
nutely described, I am greatly mistaken if North Pembroke does 
not present phenomena almost completely analogous to those of the 
mineralized Lower Silurian rocks north of Builth in Radnor- 
* See Observations in Address of last year on Mr, M‘Lauchlan’s researches. 
(Phil. Mag. S. 3. vol. xx. p. 550.] 
