514 
GEOLOGY OF THE FOURTH DISTRICT. 
nation, however, reveals the fossils which characterize this group at the east, and also at the 
same time some obscurely similar lithological characters. Similar lithological changes, 
accompanied by like changes in fossils, occur in more limited districts within the State of 
IN ew-York. 
The most marked and important changes, however, appear to be in the higher rocks of the 
New-York system. The Hamilton group and Marcellus shale, which in New-York have a 
thickness of one thousand feet, have diminished to one hundred where last examined; and 
from being the group most prolific in fossils, as it is in New-York, it has become entirely 
barren of them. The rocks forming the Portage and Chemung groups, which in their greatest 
development in New-York are scarcely less than three thousand feet in thickness, and in 
Pennsylvania much more, have, in Indiana, diminished to as many hundred. The upper of 
these groups, from being extremely fossiliferous, has become almost destitute of these charac¬ 
ters ; so that, at the furthest extreme examined, they furnish but an equivocal guide. In these 
groups, lithological character is more persistent than fossils, and it requires a knowledge of 
the superposition to identify them satisfactorily. The greater thickness of these sedimentary 
deposits, and the greater development of fossils occurring at the same point, proves the organic 
forms to have flourished in a litoral position ; and beyond these points, where the thinning of 
the strata indicates a greater 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, Michigan, Illi¬ 
nois, 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 deposition of the Helder- 
berg 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, appararently, similar depth of water, are required to give existence or continuation to a 
uniform fauna; and when 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 them¬ 
selves as far westward 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 con¬ 
tinents, the forms may be similar — we know not but they are; still, who can say what 
changes may occur, or whether any exist in the depths a thousand miles from land ? From 
