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step in the progress of geological science, not merely as elucidating 

 the history of a portion of the sedimentary formations of our island, 

 but as fixing the characters of a succession of normal groups to 

 which the strata of other parts of Europe, and perhaps of America, 

 may be referred. A large and beautifully illustrated treatise, in 

 which he intends to give a detailed description of his original ob- 

 servations and views, will soon be published. In the mean time we 

 have tasted, as it were by anticipation, the fruits of his labours, 

 having, year after year, received at our meetings the earliest intel- 

 ligence of his discoveries, and having freely discussed and criticized 

 them long before it has been possible for him to lay the whole in a 

 matured and digested form before the public. You are aware that 

 the system of rocks, which have been the chief object of his re- 

 search, constitutes the upper part of what was formerly called the 

 transition or greywacke series. In these strata, which had previ- 

 ously remained in a state of obscurity and confusion, he has distin- 

 guished several formations. The old red sandstone rests conform- 

 ably on the uppermost of these, while the lowest of them repose 

 both conformably and unconformably on the ancient slate-rocks of 

 Wales. Mr. Murchison proposes the general name of "Silurian" 

 for this whole system, as the strata may best be studied in those 

 parts of England and Wales once occupied by the ancient British 

 nation the Silures. 



The necessity of a new term has arisen from the uncertain lati- 

 tude with which the word " transition " had been applied, some au- 

 thors including in it the carboniferous rocks, and also from the still 

 greater confusion introduced by the word " greywacke," a term which 

 can only be employed conveniently, in a mineralogical sense, to 

 designate a peculiar kind of rock which has been formed at many 

 successive epochs. Thus, for example, in the memoir now under 

 review, it is shown that in Pembrokeshire grits, which have passed 

 for greywacke, occur in the true coal-measures, in the old red sand- 

 stone, in the Silurian, and in the still older systems of rock. 



Below the Silurian strata are slate-rocks of older date, in which 

 traces of organic remains have been again detected ; and Professor 

 Sedgwick has suggested the name of Cambrian for this more ancient 

 system, which is conterminous over a wide territory with the Silu- 

 rian formations, the relative position of both being clearly seen. 



Mr. Murchison has recently traced the Silurian system running in 

 zones through Pembrokeshire, and there rising out in the coast 

 cliffs from beneath the old red sandstone as conformably as in the 

 interior of the country, — an important verification of the accuracy 

 of his previous determinations. Great lithological changes are, how- 

 ever, observed to take place in these localities, so distant from the 

 best types of the system ; thus, the " Ludlow and Wenlock" formations 

 are no longer distinctly separated by subordinate limestones, and 

 are therefore simply termed the " upper Silurian rocks," and these, 

 changing their soft argillaceous characters of " mudstone," become 

 hard sandstones, yet contain some well-known organic remains ; 

 whilst the lower Silurian rocks, or Caradoc and Llandeilo formations, 



