Xtlii WTKODUCTION. 



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

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

 it 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, neveriheless, 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 known, 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 consequently 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, the forms of Ophileta,i Scalites, Raphistoma and Macli^rea, which are 

 typical of the preceding strata, have not been observed. 



When 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- 

 UHG, as well as from other 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, that the zoological relations of the lower strata in Ireland are more analogous to 

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



* Sec Geologi/ of Siutia in Europe and the Ural Mountain», by Roderick I. MtrRCHisoN, Edouard de Ver- 

 XKUIL ind Count A. voit Keyierung, Vol. i, pp. 27 &. 27*. 



t At page 11 of this Report, I have remarked that the OphUeta cemplanata, from its usually rare occurrence and 

 obaenre character*, cannot be regarded as of much importance as a characteristic fossil. Mr. Vanuxem has since in- 

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

 numbers and excellent preservation, holding the same position aa elsewhere in the State. It is probably, therefore, 

 •Btitlcd to more consideration than I had before attached to it. 



t Otologieal Report on LotKdonderry, and Parti of Tj/rone and Fermanagh, by J. 'E. Portlock, F.R.S., F.G.8.» 

 *«. Dablin li London, 1S43. 



