1846.] MURCHISON ON THE SILURIAN ROCKS OF SWEDEN. 37 



sediments of those periods alternate frequently with various erup- 

 tions of igneously formed matter. 



Comparing the lowest sedimentary masses of Russia and Scandi- 

 navia containing organic life, wherein no contemporaneous eruptive 

 matter has been deposited, and those of Great Britain in which such 

 matter abounds, and taking into consideration the great diversity of 

 their lithological characters, it seems indeed to be truly remarkable, 

 not that many species should be respectively peculiar to each country, 

 but that so many highly characteristic groups of fossils of that early 

 age should have co-existed in Russia, Scandinavia, North America 

 and Britain. 



Geologists have long ago admitted that the presence of certain Bra- 

 chiopoda afford one of the surest base-lines for the identification of 

 distant deposits ; and knowing as I do that amid the numerous forms 

 in the lower limestone of St. Petersburg there is no one more abundant 

 than the Orthis calligramma, and that this form is also equally typical 

 of the Lower Silurian simple-plaited Orthidae, and that it is indeed this 

 very species and its congeners which most abound in the Snowdon 

 slates, I adhere to the opinions expressed in the anniversary discourse 

 addressed to the Geological Society in 184'S, and reiterated after much 

 further observation in the work upon Russia, that there is no fossilife- 

 rous stratum ofhigher antiquity than the published Lower Silurian type, 

 whether the appeal be made to Scandinavia, to Russia, or to America. 

 I have indeed in a former memoir given the chief results of a 

 comparison of the most ancient fossils in these different countries, 

 and have shown to the Society, that over tracts much larger than the 

 British Isles, there isastrong and positive coincidencein all thesestrata 

 (however different in mineral sturcture), which, reposing on crystal- 

 line rocks void of organic remains, constitute, in my opinion, the first 

 recognizable chapter in the history of primaeval life ; — that period, in 

 short, in which the strata are traceable downwards into beds charged 

 with fucoids only, and which followed upwards abound in these 

 former, which characterize the typical Lower Silurian rocks of Britain 

 and other regions. 



And here I would observe, that every year of additional re- 

 searches, even in our own country, has led to the confirmation of 

 this view. In the Lower Silurian rocks of Scandinavia and Russia, 

 for example, in which calcareous matter abounds, those earliest 

 crinoids, the Cystidea, occur in myriads ; and if a geologist, argu- 

 ing on negative evidence, had been disposed to consider the Lower 

 Northern group as distinct from the Lower British group, he 

 might have dwelt on these and other apparent zoological excep- 

 tions ; and in addition to Lower Silurian he might have insti- 

 tuted a " Petropolitan system." But the researches of our Go- 

 vernment geologists under Sir H. De la Beche have found these 

 very Cystidea in a mass of rock in Pembrokeshire, formerly de- 

 scribed by myself as Lower Silurian *. By the labours of other 

 officers of the same corps, I am informed that these fossils have been 



* Whilst this memoir is going through the press, Professor E. Forheshas shown 

 to me specimens of Cystidea and Illaenus in the limestones of Bala, associated 

 with typical forms of Orthis, Trinucleus, &c. of the Lower Silurian rocks. 



