HAMILTON SHALES. 183 



of the septaria ; and probablj' the hxtior will increase in value and inijiortnnce, when it is 

 known that they make the true Roman cement. 



Thickness of the Marcellus slate. As this rock is not clearly delined in iis upward passage, 

 but is merged in the dark grayish shales of the Hamilton group, its thickness is not deter- 

 mined. It is prol)ably not less than one hundred feet at Schoharie and JManlius ; in the 

 middle and western counties, it hardly exceeds fifly feet.* 



Jlgricultural 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, eflervesces 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, conlirni 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 whicli we are obliged to make our 

 examinations, and partly from the similarity of the masses themselves. The IMarcellus 

 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, b3r 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 demarkalion 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 



♦ Hall's Report, p. 159. 



t I have changed the word group into shales, as will be seen by tlie 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 ditference between a slate and shale simply is the predominance 

 of sandy materials in the latter over the argillaceous. In conseciuence 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. 



