1846.] MURCHISON ON THE SILURIAN ROCKS OF SWEDEN. 39 



that some of the lowest fossiliferous strata of North Wales are charged 

 with shells which are well-known types of the Lower Silurian strata 

 in Britain, Scandinavia and Russia, I object quite as strongly to 

 another suggestion of Professor Sedgwick, that the Wenlock for- 

 mation may perhaps be merged into the Lower Silurian. This sug- 

 gestion however is only sustained on the ground that a few species of 

 fossils are found to pass relatively upwards or downwards between 

 the Wenlock shale and the upper beds of the Lower Silurian group 

 as hitherto defined. This fact was to a certain extent known to me 

 when I published the ' Silurian System.' I then knew (as may be 

 seen in my tables) that certain species ran from the lower group even 

 high into the upper * ; and subsequent researches of others have, I 

 admit, extended the phaenomenon. Without an acquaintance with 

 this fact, I should, indeed, have been most unfortunate in the selec- 

 tion of the name " Silurian," as embracing the lower and upper 

 groups in one system of deposit. The question then really is, whe- 

 ther after types have been recognized, and after they have been 

 applied and found to hold good over large regions of the globe, it is 

 permissible to make changes of geological demarcation founded on 

 the observation of certain slaty districts of Britain, where I venture 

 to say the palaeozoic order could never have been worked out had 

 not the clear Silurian types been previously established ; — tracts, 

 also, in which little or no continuous limestone occurs, and where 

 the whole of the Silurian series assumes to a great extent a common 

 impress. 



Looking to his native hills as he may well do with pride, because 

 he has so well unravelled their intricate relations. Professor Sedg- 

 wick would seem to suggest, that the Upper Silurian group should 

 be exclusively confined to the equivalents of the Ludlov/ rocks as 

 developed in the coarse slates of Westmoreland, &c. But although 

 this may be a good local division in the Lake country, it would, 

 I must say, be utterly valueless if tested in the region of Siluria 

 (where the rocks are unaltered and not in a slaty condition), and 

 if possible, still more so when applied to the Upper Silurian strata 

 of Norway and Sweden. 



The Wenlock limestone is, I assert, the true and only definable 

 centre of that which I have designated " Upper Silurian," and exten- 

 sive European researches, and comparisons with America made by 

 native authors as well as by Mr. Lyell, have confirmed me in this 

 view, to which I hold as an essential and fundamental point in 

 sustaining the Silurian classification. If we deprive the Upper Silu- 

 rian group of the Wenlock division, and reduce it to the Ludlow 

 rocks, it becomes in many tracts of the globe a mere shred or way- 

 board, though it be a rock of great thickness in Westmoreland f. 



* About 5 per cent, of the Silurian species were then shown to be common 

 to Lower and Upper Silurian rocks. 



t I may here state, that I have twice traversed Westmoreland and the adjacent 

 country since the publication of tlie ' Silurian System.' The first visit was made 

 before the memou's of Mr. James Marshall, Mr. Shai-pe and Professor Sedgwick 

 brought the strata there into accordance with different members of the Silurian 



