1846.] Agricultural Geology of Onondaga County. 169 



stone which it incloses. It is gray, crystalline, thick-bedded or 

 massive, and filled with organic remains. It is the well known 

 limestone which extends in a belt from the Hudson to Lake Erie. 

 It forms by its hardness its own terrace, w^hich is marked upon 

 the north by a steep escarpment formed by the out-cropping 

 limestone, which is never entirely concealed by debris, as in the 

 more fragile rocks above and below. 



The changes which have taken place in the physical constitu- 

 tion of the rocks of this period, are equally as great as those which 

 are usually called the chemical. Magnesia, for example, is no 

 longer found as an essential element in this limestone; the silex, 

 alumine and iron have also mostly disappeared, and we have a 

 pure calcareous rock, which resists the action of the weather. 

 It is, to be sure, slowly dissolved by carbonic and the organic 

 acids, and it is broken by frosts, but the soil is no longer rapidly 

 furnished with fine pulverulent matter. Between its beds, or in 

 its seams, w^e frequently see the green impure matter which forms 

 the shales below, but only in quantities suflEicient to show that 

 the source of those matters is not entirely exhausted. This rock 

 is important as a limestone for furnishing pure quick lime, and 

 also to agriculture; but not as much in this respect as the rocks 

 below, which abound in magnesia, and such as would usually be 

 called impure limestone. 



Considered in its influences upon agriculture, we believe it is 

 less important than has usually been considered: a limestone is 

 always an excellent base for tillage, it seems to be softening or 

 at least ameliorating in its influences upon the soil; a better word 

 is drying perhaps; and this effect is due to the systems of open 

 joints which always traverse the superior beds. Sometimes, 

 however, these joints are too open, and permit the direct flow of 

 streams into them. This is the fact with many of the New York 

 limestones. Sometimes they furnish facilities for draining. 



We may now pause a moment for the purpose of considering 

 the mechanical condition of the particles composing the rocks 

 which have passed under our view. From the Niagara lime- 

 stone, upon which the red shale and marl reposes, up to the 



