ON THE MENEVIAN GROUP. 183 



or seventy new or partly described species have been found peculiar to this 

 region, and these wait for description. 



That the additions to the geology of St. David's during the last few years 

 may be more clearly understood, it is well to state what formations are now 

 known to be present in the promontory that forms the north shore of St. 

 Bride's Bay. The Sui'vey Map does not profess to subcUvide the slate-rocks 

 and volcanic grits, but only to separate these as a mass fi'om the purple aud 

 grey sandstones coloured as Cambrian rock (the Harlech group of Professor 

 Sedgwick). 



In ascending order we have then : — 



1. The Harlech Group (Sedgwick), consisting of purple and greenisli-grey 

 sandstone, passing above into grey sandstones (not grits) which arc fossili- 

 feroiis. 



This mass of piirple and greenish-grey flags forms the axis of the promon- 

 tory, thrown into a violent anticlinal along the line of the so-called 

 " syenite," which is partly altered Cambrian rock, and partly crystalline 

 rock, perhaps syenite in some places. 



2. The Menevian Group, 1500 or 1600 feet thick, of dark-grey (and even 

 black) flags and shales, alternating in its upper and larger portion with sand- 

 stone. This is the great fossUiferous group. 



3. Ffestiniog Group (Sedgwick). — Hard siliceous sandstone with grey flaky 

 slate, containing LinguleUa Davisii. 



4. Tremadoc Group. — Sandstones and earthy slates, much like those of 

 the Ffestiniog group, but of a bluish-grey colour and more uniform texture. 



5. Arenig or Sliddaw Group. — A thick series of iron-stained slates and 

 flags, interlined throughout by felspar hues and felspathic ashes, containing 

 large Trilobites and shells of new species, and Graptolites like those of 

 Skiddaw. 



6. Llandeilo Flags. — Black slates, with felspar beds and interbedded trap, 

 fossils abundant. 



"We may now give a few explanatory details, as brief as possible, respecting 

 each of those formations : — 



1. Concerning the Harlech group, it must be noted that the sections on 

 the south coast (Caerbwddy for instance) show a distinct passage down- 

 wards into the central syenitic mass, so gradual as to induce the belief that 

 the mass of rock is no other throughout than altered Cambrian beds. jSTear 

 St. David's the same thing is visible. Altered purple beds close to the town 

 are succeeded by rock apparently crystalline, but showing distinct rounded 

 grains cf quartz (as minute pebbles) throughout the mass. It is e\-idently 

 a slightly altered grit or conglomerate. In other places further east, the 

 rock is certainly melted in its central portions, but passes so gradually into 

 olive shales thi-ough hardened flinty beds and horustones, that no true 

 boundary can be traced. 



The series of sandstones, purple, green, and grey, which succeed arc laid 

 down with tolerable accuracy on the Survey Map (except where bounded by 

 the prevailing faults). All the accessible localities have been searched for 

 fossils, but with no success until the highest portion was reached ; immediately 

 above the topmost purple bands, 160 feet or so of these top beds contains 

 fossils, not many species, but of the geneva, Faradoxides, Conocoryphe, Ag- 

 nostus, Them, and a new genus, Oyrtotheca. 



These are of much the same character as those of the next group, though 

 the Trilobites differ specifically. 



2. Menevian Group. — The list of fossils belonging to this group is now 



