INTRODUCTION. 3 



It is of much interest that this early sediment presents such a uniform, 

 even monotonous physical character, over the wide areas in which it has 

 been investigated in New- York, Canada, Pennsylvania, Virginia, Iowa, 

 Wisconsin, and Minnesota. 



The proportion of calcareous matter in the Potsdam sandstone at the 

 west and northwest, is much greater than in its eastern localities. This is 

 particularly manifest in some parts of the trilobite beds, where the rock 

 has often, for several feet in thickness, the character of a silico-argillaceous 

 limestone. While in its eastern localities, the Potsdam sandstone is usually 

 a hard and compact rock, enduring the action of weather in a very high 

 degree, it becomes at the west a friable, sometimes incoherent mass ; and 

 though presenting high perpendicular cliffs, surmounted and protected by 

 the cherty beds of the next succeeding formation, it is usually unfit for 

 all economical purposes, crumbling into fine sand on exposure to the frost 

 and sun. 



From what we already know of this rock in the west, we are prepared 

 to believe that some more fortunate localities will yet furnish numerous 

 and satisfactory examples of its fauna. Thus far, the trilobites are frag- 

 mentary ; the character and condition of the beds in which they occur 

 point to westerly or southwesterly currents, by which they were brought 

 to their present position. In these directions, therefore, we may probably 

 look for the highest evidences of the characteristic fauna of this period. 



I should not omit to mention, that shells of Brachiopoda and crinoidal 

 columns have been found in this sandstone in the northwest*. None of 

 these have fallen under my observation ; and from the fact that they are 

 not figured and described in Dr. Owen's Final Report^ we may infer that 

 they were in too imperfect a condition to be specifically recognized. 

 Among the thousands of Lingula which I have examined from the beds 

 on the St. Croix river, I have discovered no trace of any other shell. 

 Indeed the character of the sediment generally is such as would apparently 

 preclude the existence of other brachiopodous molluscs. 



 See OwKK'a Report, 1848, page 15; and 1862. page 499. 



