76 



Finding no fossils in the few visits which I made to the west of that 

 barrier in Caermarthenshire, wliich was provisionally agreed upon, 

 and to the north and west of which all the country was ultimately 

 to be explored by my friend Professor Sedgwick, we both of us be- 

 lieved, that such tracts, for the most part without fossils, were of 

 higher antiquity than the Silurian districts, and that, rising up from 

 beneath, they might hereafter be found to contain other and distinct 

 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 I'ocks 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 tlie 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 altei-ed by 

 the intermixture of trappean rocks, both of contemporaneous 

 origin and subsequent intrusion. But in these altered rocks the 

 Ordnance surveyors have detected true LoAver 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 public, 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 contemporaneous 

 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- 



* See Observations in Addressof last year on Mr. M'Lauchlan's researches. 



