172 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jail. 6, 



which threw down thick sheets of porphyry, trappean ashes and other 

 igneously formed matter on the bottom of the then sea, and that in 

 relation to them, the quiet sedimentary deposits containing animal 

 life were small. By such operations and by the subsequent eruption 

 of other igneous rocks, the whole series was, in these tracts, greatly- 

 expanded and diversified, whilst by other events the masses were 

 thrown up into lofty mountains and underwent much crystallization. 

 But then comes the question, what are the animals which lived during 

 the accumulation ol' this extravasated and troubled marine series? 



Assuming that Professor Sedgwick be perfectly correct in his iu- 

 terpretati(m of what are really the lowest strata (though as yet we 

 have no proofs like those derived from Scandinavia, that ihehypozoic 

 rocks of Anglesea are inferior to the lowest fossiliferous beds), we 

 are informed by him, that in passing from W. to E. on several par- 

 allels, whether near Tremadoc, near Penrhyn, or again near Cader 

 idris, there exists an ascending series from lower slaty and quartzose 

 strata through igneous rocks in which no fossils have been observed, 

 and thence into beds charged with a Lingula and fucoids ; and that 

 after passing through other alternations of slaty and igneous rocks, 

 the geologist reaches the chief fossiliferous zone of the north-western 

 portion of Wales, as exhibited in the range of Snowdon. 



He also points out that the same beds with Lingula occur along the 

 Merioneth anticlinal in the valley of Festiniog, and that in proceeding 

 westwards therefrom, a grand succession is seen up to the well-known 

 horizon of Bala. Admitting that the Lingula of these low beds in 

 North Wales is a new species, that single circumstance cannot surely 

 be of much value in a discussion like this. Lingulae occur in nearly all 

 the formations or subdivisions of the Silurian system of Britain from 

 the Ludlow rocks to the Llandeilo flags, whilst in North America, 

 as in North Wales, they are found at the base of the whole of the same 

 series of animal life. Again in Russia, where fucoids only have been 

 detected in the lowest strata, the shells immediately above them are 

 Orbiculas and Ungulites (Obolus), both of them, like the Lingula, 

 small hortiy bivalves suited to a sandy or muddy sea-bottom in which 

 there had been little calcareous matter. In North Wales, as in other 

 palaeozoic countries, the true test of the age of the rocks lies in the 

 lowest zones in which the common and characteristic fossils are found. 

 Now among the lowest of these North Welsh beds Professor Sedg- 

 wick mentions the Asaphus Powisii, a crustacean first described by 

 myself from the very uppermost beds of the Caradoc sandstone of 

 Shropshire, or rather from the Horderley limestone at the base of the 

 Wenlock shale, its associated fossils near Tremadoc being Graptolites 

 Murchisonce and G.foliaceus, both common Silurian forms. A Ho- 

 malonotus is next cited as pertaining to still higher beds, and is 

 identified with a species from the Lower Silurian beds of Witting- 

 slow in Shropshire. 



Tlie Leptcena sericea, a species frequent in the Wenlock shale, and 

 which also ranges, as I have shown, throughout the Lower Silurian 

 rocks, is said by Professor Sedgwick to run low down into the Snow- 

 donian group together with the Trinuckus Caractaci, the most typical 



