154 Sir Roderick I. Murchison 07i the 



which explained the first clear succession downwards from the base 

 of the old red sandstone. They will not, I trust, forget the toil by 

 which these results were obtained, nor cease to be alive to the fact, 

 that, by the forms which I described, order was at length elaborated 

 amidst various slaty tracts, often highly dislocated, and in parts 

 metamorphosed, over which such Silurian types are now found to 

 spread. For, even in reference to Britain, the succession of the 

 broken and porphyritic region of North Wales, Westmoreland, and 

 Cumberland, might, I apprehend, have long remained undeciphered, 

 if it had not been for the constant appeals which the geologists who 

 have explored those tracts have made to the established Silurian 

 strata. Is then the key which has served to open out such regions 

 to be now thrown away "? 



But passing for a moment to some of its leading details, let us now 

 see if the memoir recently read by Professor Sedgwick contains any- 

 thing new to lead us to change the previous arrangements. Enlarging 

 the discovery of Mr Davis announced to us in. 1845,* of the exist- 

 ence of a species of Lingula in the rocks near Tremadoc in Carnar- 

 vonshire, Professor Sedgwick tells us, that this band lies many thou- 

 sand feet beneath the Bala limestone, including, however, it will be 

 observed, enormous interpolations of stratified igneous rocks. 



I grant that his sections (obscure as he admits some of them to 

 be) afford proof of an enormously thick succession of " great physi- 

 cal masses." They clearly indicate, that in the early days of sub- 

 marine life, the area of the earth's surface now constituting Merio- 

 neth and Csernarvonshire, was much agitated by plutonic eruptions 

 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 of this extravasated and troubled marine series ? 



Assuming that Professor Sedgwick be perfectly correct in his in- 

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

 have no proofs like those derived from Scandinavia, that the hypozoio 

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

 informed by him, that in passing from W. to E. on several parallels, 

 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 



* Quart. Journ. Gcol. Soc, vol. ii., p. 70. 



