518 
GEOLOGY OF THE FOURTH DISTRICT. 
The Utica slate (and perhaps the upper part of the Trenton limestone) may be regarded 
as an equivalent of the Llandeilo flags, mostly from similarity of lithological character; but 
one or two fossils being known in New-York, which are identical with those of the latter for¬ 
mation. 
The formations below the Trenton limestone appear not to be recognized in England and 
Wales, with the same distinctness, or in the same degree of development as in New-York. 
They are probably equivalent to some of the rocks included in the Cambrian system, and 
which doubtless will yet be more fully recognized. 
That there should be important differences in the character of sedimentary deposits, at 
such great distances, is not surprising; indeed, it is more astonishing that there should be 
so great a degree of similarity, when we consider the circumstances under which they were 
produced. It has been shown, in the last chapter, that upon our own continent, within the 
distance of a few hundred miles, great changes occur in the nature of the sedimentary strata; 
changes which almost prevent a recognition of the same formations, and which are at the 
same time accompanied by a different association of organic remains, showing that the nature 
of the sediment influenced the character of the living forms. Absolute similarity, therefore, 
in all the lithological and fossil characters is hardly to be expected in any two points at re¬ 
mote distances from each other. 
The differences which the same strata present at distant points, and the comparative degree 
of development which formations exhibit, have given rise to a diversity of nomenclature, in 
different parts of the United States. The surveys in progress required the adoption of pro¬ 
visional names at least, and these have been given, generally according to the nature of the 
rock, or from some locality where its characters were best developed. From the different 
Reports I have constructed the following Table, which may, perhaps, be of use in comparing 
the formations of different parts of the country, and in reading the respective reports. 
