Xviii INTRODUCTION. 



That part, of my work already completed in detail, shows a more perfect similarity with 

 the Lower Silurian strata of Europe than we had heretofore supposed ; and though there 

 is not a large number of identical species, the analogy of others is quite interesting and 

 satisfactory. 



In our extreme lower formations, we have found little that can be positively identified 

 with European deposits ; still, the Potsdam sandstone, our oldest fossiliferous rock, appears 

 to hold, in the scale of formations, a similar place with the Ungulite grit of St. Peters- 

 burgh,* which is there charged with fragments of Obolus, giving it the almost micaceous 

 aspect which the Lingula does to its analogue in New-York. The paucity of organic forms 

 at this period, and in the two succeeding formations, would lead us not to expect a very 

 complete analogy between groups of this age in countries widely separated from each other. 

 We have, nevertheless, in these extremely ancient deposits, a few forms which are 

 peculiar, and sufficiently characteristic to identify the strata in which they occur. These 

 forms do not appear to have been recognized elsewhere ; or if Icnown, no great importance 

 has been attached to them. It is probable, however, that in Europe, as in many parts of 

 the United States, the Calciferous sandstone and Chazy limestone are not well developed, 

 and consecpiently their typical fossils do not exist. We find ourselves, therefore, forced to 

 commence our comparison with European formations, from the Trenton limestone; in 

 which, thus far, tlie forms of Ophileta,i Scalites, Raphistoma and Machp-ea, which are 

 typical of the preceding strata, have not been observed. 



Wlien we compare our species of the lower groups with those of Europe, as made known 

 to us, first, by the publication of the Silurian System of Mr. Murchison, and subsequently 

 in the work already cited, by the same Author, E. de VERNEUiL-and Count von Keyser- 

 LiNG, as well as from otiicr sources, we find a very instructive and interesting analogy. 

 The Geological Report of the Ordance Survey of Ireland, by Capt. Portlock,! has like- 

 wise shown us other forms identical with our own ; and we are able to prove, from this 

 author, iliat the zoological relations of the lower strata in Ireland are more analogous to 

 liiose of New-York and tlie United States, than are those of the same age in England and 



* Sec Geology of Rtissia in Europe and the Ural Mountains, by Roderick I. Murchison, Edouard de Ver- 

 NEUiL and Count A. von Keyserling, Vol. i, pp. 27 & 27*. 



t At page 1 1 of this Report, I have remarked that the Ophileta complanata, from its usually rare occurrence and 

 obscure characters, cannot be regarded as of mucli importance as a cliaracteristic fossil. Mr. Vanuxem has since in- 

 formed me, that in a late geological excursion in the vicinity of Lake Cliamplain, he has found this fossil in great 

 numbers and e.^ccllent preservation, holding the same position as elsewhere in tlie State. It is probably, therefore, 

 entitled to more consideration than 1 had before attached to it. 



t Geological Report on Londonderry, and Parts of Tyrone and Fermanagh, by J. E. Portlock, F.R.S., F.G.S., 

 ii.c. Dublin it London, 1813. 



