HAMILTON GROUP. 
151 
end of the district: the arrangement adopted, though less satisfactory, is, from its simplicity, 
of more ready application, and subserves all present purposes. 
The group is of great thickness; in no part probably less than three hundred, and swelling 
to seven hundred feet. It commences near the Hudson, and extends to Lake Erie. It is 
therefore important from its thickness, and for the extent of surface which it occupies in New- 
York. It consists of shale, slate and sandstone, with endless mixtures of these materials. 
They form three distinct mineral masses as to kinds, but not as to superposition or arrange¬ 
ment, though generally the sandy portion is the middle part of the group. 
The first in the order of tenuity of particles, is rather a fine-grained shale, often fissile or 
slaty; color some shade of blue, usually dark or blackish. 
The second is a coarse shale, often mixed with carbonate of lime. Its color blue or dark 
grey when fresh, but becomes of an olive or brown by long exposure to the weather: the 
color due to manganese. No tendency to separate into regular layers whatever; but where a 
mass has been long exposed, it shows numerous curved divisions, the curves very short and 
irregular, and the parts horizontally arranged. 
The third kind is not so common as the two first. It is a well characterized sandstone, 
but more or less mixed with either of the two others. It is often in layers, though rarely 
straight, and usually short or interrupted; sometimes mixed with carbonate of lime. The 
colors of this kind are more various ; brown of various shades, olive, greenish and yellowish. 
The group generally is deficient in building materials, the shale of the first kind readily 
crumbling by exposure to the air; the two latter kinds alone furnish building stone. The 
best is where limestone forms the cement, and sand is in the greatest abundance. So rare is 
the occurrence of regular layers in the group, that their absence is a good negative character 
of it, and its brownish color externally or where weathered, a good positive one of the group 
generally. 
It abounds in fossils, such as shells, corals, trilobites, fucoids, and a few plants resembling 
those of terrene origin. It is admirably characterized by its fossils ; numerous species, and 
even genera, commencing with the group and ending with it. In organic remains, it is the 
most prolific of all the New-York rocks. Among the numerous fossils by which the group 
is readily recognized, are the three figured in wood-cut 36 ante. 
No. 1. Head of De Kay's dipleura (D. dekayi), that part being figured from its number and 
characteristic form, the entire ones being very rare. The head in some measure resem¬ 
bles that of the Dolphin-head trimerus of the Clinton group, or more closely the one in 
the Niagara sandstone; but the difference is considerable in the form of the snout, and 
the parts around the eyes being more full or protuberant in the dipleura. The genus 
Homalonotus, figured in Mr. Murchison’s work, and which occurs in the same position 
in England, resembles the dipleura, but differs from it in the tail being divided into three 
lobes or parts as in the Trimerus, while the Dipleura is unilobed; in other respects, 
there is something like identity. In Ladd’s quarry on the Chenango canal, north of 
Sherburne village, I found on a thin slab two or three small perfect trilobites, with the 
same structure of the tail which belongs to the English genus, associated with the 
