Oh. XXVn.] POTSDAM SANDSTONE— UNITED STATES. 531 



this age, and how devoid of life they are for the most part in North 

 Wales, Ireland, and Shropshire, the information respecting such 

 minute details of the Natural History of these crustaceans, as is sup- 

 plied by the Bohemian strata, may well excite our astonishment, and 

 may reasonably lead us to indulge a hope that geologists may one 

 day gain an insight into the condition of the planet and its inhabit- 

 ants at eras long antecedent to the Cambrian ; for those areas which 

 have been subjected to a scrutiny as rigorous as North Wales and 

 Bohemia form truly insignificant spots on a map of the whole globe. 



In Bohemia the primordial fauna of Barrande derived its impor- 

 tance exclusively from its numerous and peculiar trilobites. Besides 

 these, however, the same ancient schists have yielded two genera of 

 brachiopods, Orthis and Orbicula, a pteropod of the genus Theca, 

 and four echinoderms of the Cystidean family. 



All the Bohemian species differ as yet from any found in England, 

 which may be due entirely to the influence of geographical causes. 

 It seems, nevertheless, to confirm the view here taken, of the " pri- 

 mordial zone " being cbaracterized by fossils distinguishable from the 

 whole Lower Silurian group ; because the other and higher Silurian 

 formations of Barrande have each of them several species in common 

 with the successive subdivisions of the British series. 



Sweden and Norway. — The Upper Cambrian beds of North Wales 

 are represented in Sweden by strata, the fossils of which have been 

 described by a most able naturalist, M. Angelin, in his " Palseonto- 

 logica Suecica (1855-'4)". The "alum-schists," as they are called 

 in Sweden, resting on a fucoid-sandstone, contain trilobites belonging 

 to the genera Paradoxides, Olenus, Agnostics, and others, some of 

 which present rudimentary forms, like the genus last mentioned, 

 without eyes, and with the body segments scarcely developed, and 

 others again have the number of segments excessively multiplied, 

 as in Paradoxides. These peculiarities agree with the characters 

 of the crustaceans met with in the Upper Cambrian strata, before 

 mentioned. 



The Swedish rocks have also yielded crustaceans of the family 

 Cytherinida, and among the mollusca a small species of Orthoceras, 

 the only primordial cephalopod yet known, and also a Graptolite, 

 together with most of the fossil forms discovered by Barrande in the 

 Bohemian strata of the same age. 



United States and Canada. — In the table at p. 570, I have already 

 pointed cut the relative position of the Potsdam Sandstone, which 

 has long been supposed to be the lowest fossiliferous formation in the 

 United States and Canada. The late Dr. Dale Owen published in 

 1852 a graphic sketch, in his survey of Wisconsm, of the lowest 

 sedimentary rocks near the head-waters of the Mississippi, lying at 

 the base of the whole Silurian series. They are many hundred feet 

 thick, and for the most part similar in character to the Potsdam sand- 

 stone above described, but including in their upper portions interca- 



