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formations can be here defined; the latter containing many well- 

 known fossils. The lower Silurian rocks are still more largely de- 

 veloped ; a vast thickness of fossiliferous sandy strata being quite 

 identical with the " Caradoc sandstones," whilst the Llandeilo flags 

 with Asaphus Buchii and A. Bigsbii (a new species of the author) 

 occur in the haven called Moseley-wick Mouth. This coast section 

 is 150 miles distant from the N. eastern extremity of the Silurian 

 system. 



The Cambrian Si/stem. — If divided by a line passing from E. to W., 

 the northern half of Pembroke is exclusively composed of the older 

 rocks of the Cambrian system, consisting, in descending order, of 

 a. Dark-coloured incoherent schists, with few stone bands, no cal- 

 careous matter, and scarcely any traces of organic remains. 

 These occupy a great breadth, and, as in Caermarthenshire, 

 they form the beds of passage between the Silurian and the 

 Cambrian systems (sometimes without any break). 

 h. Hard grits and flagstones, coming strictly within the definition of 

 greywacke of German mineralogists. 



c. Hard purple sandstones and schists, identical with the slaty grey- 

 wacke of the Longmynd, Salop, (the Lammermuir hills, Scotland, 

 may be cited as a good and well-known type of these rocks). 



d. Slates coarse and fine, with quartz veins and concretions. 



At St. David's, Pantiphilip, and Scillyham, where the roofing-slates 

 are quarried, the author has detected what he believes to be a coin- 

 cidence between the laminae of deposit as indicated by differently co- 

 loured layers of sediment, and the lines of slaty cleavage ; though in 

 the great majority of cases in Pembroke the rocks of this system, 

 whether consisting of sandstone, schist, or hard slate, exhibit the di- 

 vergences between the lines of true bedding and slaty cleavage, 

 so clearly ,and ingeniously explained by Professor Sedgwick. The 

 author therefore thinks it right to point to these exceptions to the 

 observations of Prof. Sedgwick, because if strictly scrutinized the 

 phaenomena are not placed in opposition to them, since it was his 

 belief upon the spot, that the crystallizing action which gave to these 

 masses their hard slaty properties produced the flaglike laminae of the 

 beds. 



Trap Rocks. — Of these there are distinctly two classes : 1 , Bedded, 

 and synchronous with the formation of the older rocks'j 2, Posterior and 

 intrusive. Of the former there are no examples like those cited in West 

 Salop, Montgomery, and Radnor, (see former memoirs,) of alternation 

 with the strata of the Silurian system, being all confined to the Cam- 

 brian rocks. The tract extending from Fishguard to St.David's and the 

 Isle of Skomer ofter illustrative examples of both these classes of trap. 

 In addition to the varieties of sienite, compact felspar rock (corneen 

 of De la Beche), greenstone, &c., of which these rocks are composed, 

 the author has detected crystallized chromate of iron and albite in St. 

 David's Head, — small veins of copper ore also occur between Solfacb. 

 and St. David's. Among the more remarkable changes effected by 

 the intrusive trap, he adverts to jaspidified schists inclosed between a 

 large bifurcated mass of trap proceeding from Trafgarn. Havingtraced 



