290 IDENTIFY OF THE WESTERN FORMATIONS 



examined, they furnish but an equivocal guide. In these groups, 

 lithological charaeter is more persistent than fossils, and it requires 

 a knowledge of the superposition to identify them satisfactorily. 

 The greater thicknet^s of these sedimentary deposits, and the 

 greater developement of fossils occurring at the same point, proves 

 the organic forms to have flourished in a littoral position; and 

 beyond these points, where the thinning of the strata indicates a 

 gi'eater distance from the shore, the fossils diminish, and at the 

 more distant and deeper points are not found at all. There is no 

 evidence of denudation in these instances, and if there had been, 

 the parts left would have retained the same fossils — had it ever 

 contained them — as they do further east. 



Throughout that part of the ancient ocean now occupied by 

 Ohio, Indiana, INIichigan, Illinois, and even to the west of the 

 Mississippi, there appears to have been comparatively a small 

 number of living forms existing from the period of the final depo- 

 sition of the Helderberg limestones, to the commencement of the 

 carboniferous period; while in New York, during the same 

 period, there were a greater number of forms and individuals than 

 in all the preceding periods. Without desiring to diminish the 

 value of fossil characters as means of identifying strata, it must 

 still be acknowledged, that similar conditions in the bed of the 

 ocean, and, appai-ently, similar depth of water, are required to 

 give existence or continuation to a uniform fauna ; and wlien 

 we pass beyond the points where these conditions existed in the 

 ancient ocean, we lose, in the same degree, the evidences of 

 identity founded upon fossils. Some species it is true have lived 

 onward through successive depositions, often of very different 

 nature ; yet, at the same time, these may not have had a very wide 

 geographical range. In the case before us, some species have 

 lived during the deposition of all the rocks from the Hamilton 

 through the Chemung groups, and yet they have never extended 

 themselves as far westw^ard as Ohio and Indiana, although the 

 nature of the deposits there was as favorable to their existence 

 as in New York. 



For the distance of one hundred or two hundred miles from 

 the shores of the present continents, the forms may be similar, — 



