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GEOLOGY OF THE FOURTH DISTRICT. 
CHAPTER V. 
Description of the rocks and groups of the New- York System embraced within the 
Fourth Geological District. 
The previous chapter contains an enumeration of the deposits forming that part of the 
New-York System between the Primary and the Medina sandstone, or No. 9 in the order of 
succession, and the lowest rock of the district. The rocks of the Fourth district consist prin¬ 
cipally of series of limestones, shales and sandstones, each passing into the other by insen¬ 
sible gradations in some points, while in others the line of separation is distinctly marked. 
Each formation or stratum is limited on the north by its outcrop, and on the south by the 
superposition of the next successive deposit, under which it disappears. These are for the 
most part continuations of those which commence farther east. In some instances they have 
thinned partially, and some members of the Helderberg series are wanting; in others, as the 
Medina sandstone, and Niagara group, the greatest development appears within the limits of 
this district, and diminishes to the east. 
Investigations throughout the whole State have proved that there is some one point or 
limited area where each rock, or the members of each group, are better developed than in any 
other, and this one is adopted for the name. The advantage of names from localities over 
those from descriptive characters, is decidedly great. In the latter case, the description or 
character may not convey to the reader precisely the same ideas as those possessed by the 
writer ; and again, distinctions in character cannot be so accurately described, but that another 
rock may possess the same in a sufficient degree to be mistaken for the true one. The charac¬ 
ters of a rock rarely remain uniform over a great extent of country ; and hence a descriptive 
name, applicable in one place, may not be so in another. On the other hand, when a name is 
given from the locality, the description of the rock follows ; and although it may vary at distant 
points, or another rock be found possessing the same character, still the type remains ; and 
whatever of doubt or obscurity may ever arise, the examination of the original locality would 
decide all difficulties. Since rocks are identified more by their fossil contents than by their 
lithological character, a name descriptive of the latter is of less consequence than formerly, 
when fossils were the subordinate characters of a mass. 
Notwithstanding, however, that palaeontological characters have taken precedence over all 
others in distinguishing sedimentary strata, still the lithological character must not be over¬ 
looked ; for in some cases it will be found an unerring guide, if properly understood, ove 1 ’ 
