CaXXVIL] POTSDAM SANDSTONE OF N. AMERICA. 451 



One of the so-called " primordial" Trilobites of the genus Sao, a form 

 not found as yet elsewhere in the world, has afforded M. Barrande a fine 

 illustration of the metamorphosis of these creatures; for he has traced 

 them through no less than twenty stages of their development. A few 

 of these changes have been selected for representation in the accompany- 

 ing figures, that the reader may learn the gradual manner in which differ- 

 ent segments of the body and the eyes make their appearance. When 

 we reflect on the altered and crystalline condition usually belonging to 

 rocks of 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 supplied 

 by the Bohemian strata, may well excite our astonishment, and may rea- 

 sonably lead us to indulge a hope that geologists may one day gain an 

 insight into the condition of the planet and its inhabitants at eras long 

 antecedent to the Cambrian ; for those parts of the globe which have 

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

 are insignificant spots, and we are every day discovering new areas, es- 

 pecially in the United States and Canada, where beds as old as the 

 " primordial schists," or older, may be studied. 



Sweden and Norway. — The Lingula Flags of North Wales, and the 

 " primordial schists" of Bohemia, are represented in Sweden by strata, 

 the fossils of which have been described by an able naturalist, M. An- 

 gelin, in his " Palseontologica Suecica (1852-4)." The "alum schists," 

 as they are called in Sweden, resting on a fucoid-sandstone, contain 

 trilobites belonging to the genera Paradoxides, Olenus, Agnostus, and 

 others, some of which present rudimentary forms, like the genus last 

 mentioned, without eyes, and with the body segments scarcely de- 

 veloped, and others again have the number of segments excessively mul- 

 tiplied, as in Paradoxides. These peculiarities agree with the characters 

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

 tioned. 



United States and Canada. — In the table, at p. 444, 1 have already 

 pointed out the relative position of the Potsdam Sandstone, which has 

 long been known as the lowest fossiliferous formation in the United States 

 and Canada. I have seen it on the banks of the St. Lawrence in Canada, 

 and on the borders of Lake Champlain, where, as at Keesville, it is a white 

 quartzose fine-grained grit, almost passing into quartzite. It is divided 

 into horizontal ripple-marked beds, very like those of the Lingula flags of 

 Britain, and replete with a small round-shaped Lingula in such numbers 

 as to divide the rock into parallel planes, in the same manner as do the 

 scales of mica in some micaceous sandstones. This formation, as we learn 

 from Mr. Logan, is 700 feet thick in Canada; the lower portion consisting 

 of a conglomerate with quartz pebbles ; the upper part of sandstone con- 

 taining fucoids, and perforated by small vertical holes, which are very 

 characteristic of the rock, and appear to have been made by annelids 

 (Scolithus linearis). 



On the banks of the St. Lawrence, near Beauharnois and elsewhere, 



