HAMILTON SHALES. 
183 
of the septaria; and probably the latter will increase in value and importance, when it is 
known that they make the true Roman cement. 
Thickness of the Marcellus slate. As this rock is not clearly defined in its upward passage, 
but is merged in the dark grayish shales of the Hamilton group, its thickness is not deter¬ 
mined. It is probably not less than one hundred feet at Schoharie and Manlius ; in the 
middle and western counties, it hardly exceeds fifty feet.* 
Agricultural characters of the Marcellus slate. The chemical constitution of this rock, 
and the ready conversion of its materials into soil, confer upon it important and useful 
adaptations to agriculture. The rock, especially the lower part, effervesces with acids ; and 
hence the calcareous matter is in sufficient quantity to influence the soil favorably, and fit 
it particularly for wheat. In addition to the lime, it also contains carbonate of magnesia, 
which, by its presence, adapts the debris of the rock to the culture of maize. Observations 
upon the region where this rock prevails, confirm these statements. Where there appear 
to be exceptions to them, it will probably be found to arise from height, or some physical 
cause independent of composition. 
§ 3. Hamilton shales. f 
It is difficult to ascertain the point where the Marcellus slate ends, and the Hamilton 
shales begin ; partly from the circumstances under which we are obliged to make our 
examinations, and partly from the similarity of the masses themselves. The Marcellus 
slate becomes sandy, and loses its dark color, as well as its slaty character, and is conse¬ 
quently merged gradually into the shales which succeed in the ascending order. The 
Hamilton shales, however, are limited above, or superiorly, by a dark colored mass which 
has been called the Tully limestone. This would seem a sufficiently distinct limit, if the 
limestone extended eastward ; but as it is absent in the river counties, and scarcely extends 
beyond the central counties in this direction, the group is still left without a distinct line 
of demarkation in nearly one half of the State. We are, therefore, obliged to resort to a 
careful study of its fossils, in order to define the limits which the mass occupies. 
However this may be, we have, with this group, entered upon a series of rocks which 
are in the main siliceous, and in which very little calcareous or magnesian matters are to 
be found ; and hence it is that the agricultural capabilities of those sections of the State, 
where these rocks predominate, are also changed. The masses composing these shales, as 
* Halt's Report, p. 159. 
f I have changed the word group into shales, as will be seen by the several reports on the rocks of Central and 
Western New-York The change seemed to be called for, as (he name now expresses the character of the masses to 
which it is applied. In accordance with this view, I have frequently used the denomination Marcellus slate as also 
expressing the nature of this rock. It is, however, to be understood, that the word slate, or shale, is always appli¬ 
cable to a mass which may fall under our examination; for there are some slates in the Hamilton rocks, and the upper 
part of the Marcellus slate becomes a shale. The difference between a slate and shale simply is the predominance 
of sandy materials in the latter over the argillaceous. In consequence of the excess of sandy matter, shales are thicker 
bedded than slates. The two kinds of rocks, however, run into each other by insensible gradations, especially when 
the grains of sand are fine. 
