NIAGARA GROUP. 
99 
thin film of the ore. In excavating the limestone near the Whirlpool, specimens were obtained 
which were crossed by numerous thin veins, sometimes one-eighth of an inch thick. I was 
also informed, that during the excavation of the Erie canal at Rochester, several hundred 
pounds of galena were found in a single cavity. This is perhaps not improbable, and the 
celestine and gypsum sometimes occur in masses of nearly equal weight. 
In some of the geodes, and among the crystals of calcareous spar, there occur long pris¬ 
matic crystals of a dark brown color, which appear like black hairs ; they are attached to, or 
penetrating the calcareous crystals. This mineral has usually been referred to Rutile , but 
Dr. Beck regards it as Achmite. 
Iron pyrites occurs in the limestone, as well as in the shale ; but in the former, it is usually 
in nodules or irregular masses. 
Native sulphur is of common occurrence in the small cavities in the encrinital limestone at 
the Cold-spring quarries, two miles east of Lockport. 
Calcareous tufa is formed in many situations, where water, percolating through the lime¬ 
stone, finds an outlet in the shale. The deposition forms in the fissures of the rock, and finally 
accumulates in large masses projecting from the sloping edges, until from its increased weight 
it falls down. Instances of this kind may be seen in the banks of the Genesee below Ro¬ 
chester, and in similar situations at Lockport and elsewhere. 
Springs charged with sulphuretted hydrogen occur both in the shale and in the limestone, 
though not usually very copious. The localities of some of these will be enumerated under 
the respective counties in which they occur. 
Springs. — From the nature of the two rocks forming the group, the line of junction every 
where gives origin to springs, which flow over and fertilize the northern slope of the escarp¬ 
ment. The limestone, from the numerous vertical joints, is pervious to water; and toward 
its northern limit, the soil is often dried in consequence. The shale, from being impervious, 
holds up the water thus falling upon it, and from the southern dip, operates as a reservoir, 
affording a constant supply in the springs which break out along its northern edge. This 
character is more obvious in the western counties ; while farther east, from the more gradual 
slope of the shale and thinning of the limestone, the springs rarely appear on the surface, but 
the water is disseminated through the soil. 
Agricultural characters. — The two members of this group are marked, to a considerable 
degree, by a difference in the soil. The destruction of the shale has given rise to a clay, 
which, mingling with the more sandy production of the Medina sandstone on the north, has 
produced a soil of unequalled fertility ; and there is rarely, if ever, to be found a better wheat¬ 
growing soil than the portion overlying this rock. In some places it has a greater amount of 
argillaceous matter than is desirable, and forms a stiff soil; but where the slope of the surface 
is sufficient for effectual drainage, it produces no inconvenience. 
The soil covering the limestone, particularly where it is a little elevated above the country 
on the north, is of a loamy character, the argillaceous nature of the mass below having had 
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