1847.] MURCIIISON ON THE SILURIAN ROCKS OF N. WALES, ETC. 171 



The names of William Smith remained unchanged, although his 

 original mineral types were in after-times found to be often of very 

 small persistence. It was not proposed to change the name of Lias, 

 when that formation was found to be three times as thick at Whitby 

 (with numberless new inferior strata and many new fossils) as in 

 the South of England where it was first named. Modern geology 

 will stand on an insecure basis, if the principle of identifying strata 

 by their fossils be abandoned. But it is now proposed to abrogate 

 the established name of the type, and substitute for it that of a tract 

 the fossils of which have never been published, long after the Lower 

 Silurian rocks were first named and distinguished by peculiar forms 

 — when the fossils of that age have been so labelled in numerous 

 museums on both sides of the Atlantic, and when geologists and palae- 

 ontologists of all countries have adopted the term, and already know 

 (through the previous admissions indeed of Professor Sedgwick 

 himself) that the greatest portion of the Cambrian is zoologically 

 nothing more than a downward extension of the Lower Silurian 

 group. 



Let me then ask my brother-labourers to adhere to the use of 

 a name which has been so long current, and which for some years 

 has had a definite meaning attached to it both by its fossils and 

 position in various parts of the world. Geologists have already 

 honoured me with their approbation for having worked out certain 

 phaenomena, 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 7iow 

 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 unde- 

 ciphered, if it had not been for the constant appeals which the geo- 

 logists 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 aw^ay ? 



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 ex- 

 istence of a species of Lingula in the rocks near Tremadoc in Car- 

 narvonshire, Professor Sedgwick tells us, that this band lies many 

 thousand 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) aff'ord proofs of an enormously thick succession of " great 

 physical masses." They clearly indicate that in the early days of 

 submarine life, the area of the earth's surface nov/ constituting Merio- 

 neth- and Carnarvonshire was much agitated by plutonic eruptions 

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



