402 Dr. H. Sicks — Life Zones in Palceozoic Rocks. 



In 1888, after another visit, Mr. J. Evans, F.G-.S., a former 

 Manager of the Penrhyn Slate Quarry, kindly furnished me with 

 a careful statement of the succession made out in the tunnels, and 

 in working the quarry, and the following table is mainly compiled 

 from the details supplied to me by him. By its side I place also 

 a detailed statement of the succession at St. David's : — 



Nant Ffrancon, and Penrhyn Slate 

 Quarry, Carnarvonshire. 



14 Dark slates and flags (Mene- 

 vian ?) . 



13 Grits and sandstones (Bron- 

 Uwyd). 



12 Green slates. 



[Conocoryphe viola, etc.) 



11 Purple slates. 



10 Purplish hlue slates. 



9 Hard grits. 



8 Purplish blue slates. 



7 Grits. 



6 Ked slates. 



5 Purplish hlue stripy slate. 

 4 Dyke (basic rock) . 



St. David's, Pembrokeshire. 



14 Dark flags and black slates 



(Menevian) 750 



{Paradoxides, etc.) 

 13 Grey, purple, red, and greenish 

 sandstones, flags, and slates. 

 {Paradoxides, etc.) 

 12 Grits, sandstones, and greenish 



flags 



[Plutonia, Paradoxides Hark- 

 nessii, Conocoryphe Lyellii, 

 Micr or discus, etc.) 



1800 



150 





7-11 Red and purple sandstones, 

 flags, and slates .... 



6 Red slates and flags .... 

 [Olenellus fauna.) 



5. Purplish flaggy sandstones . . 



4 Dykes of basic rocks cut across 

 the Lower Cambrian rocks 

 at this and at various other 

 horizons 



3 Greenish flaggy sandstones . . 

 [Olenellus fauna.) 



2 Conglomerates 60-150 



(Many very large pebbles.) 



1 Pre -Cambrian rocks .... 

 (Granite, felstones, volcanic 

 ash, etc., etc.) 



1000 

 60 



20 



440 





3 Green hard slates. 



2 Conglomerates (with large 



pebbles). 

 1 Pre-Cambrian rocks (felstones, 



etc.) 

 As there are several faults in the 

 quarry between Nos. 3 and 10, it 

 is quite possible that some of the 

 beds have been repeated. 



It will be seen that there is a general resemblance in the 

 succession in the two areas. At first, as the sea encroached on the 

 pre-Cambrian land, the deposits would accumulate mainly in minor 

 troughs separated by the higher plateaux and mountain ridges, and 

 important differences in the thicknesses of the lowest Cambrian strata 

 would necessarily occur even within limited areas. As depression 

 went on oceanic conditions would prevail, and the sediments would 

 become more alike over extensive areas. In a paper which I 

 published in 1875,' I endeavoured to show the direction of encroach- 

 ments of the sea, and of the migrations of the lowest Paleozoic 

 faunas over the European areas, and in 1876- I entei'ed more fully 

 into this question in a discussion of the deposits as then known in 



^ Quart. Journ. Geol. Soc. vol. xxxi. p. 552. 



- " Some Considerations on the Probable Conditions under which the Palaeozoic 

 Rocks were Deposited over the Northern Hemisphere." Geol. Mag. Dec. II. 

 Yol. III. (April, May, and June, 1S7G). 



