222 PALEOZOIC TIME. 



has been found at several localities, along with other Upper Silurian species. (See, 

 further, p. 230. ) 



The color of the Niagara limestone is commonly dark bluish -gray to drab. It is 

 sometimes quite impure, and good for hydraulic purposes. A specimen from Mako- 

 queta, Jackson County, Iowa, afforded J. 1>. Whitney — Carbonate of lime 52 - 18, 

 carbonate of magnesia 42*64, — with - 35 of carbonate of soda, traces of potash, car- 

 bonate of iron, chlorine, and sulphuric acid, 0'63 of alumina and sesquioxyd of iron, 

 and 4.00 insoluble in acid, — making it nearly a true dolomite. 



Structural peculiarities. — The Medina beds bear evidence of having 

 been formed as a sand-flat or reef accumulation. Besides the thin 

 lamination alluded to, they abound in ripple-marked slabs (Fig. 62, 

 p. 83) ; mud-cracks (Figs. 64, 65), due to sun-drying ; wave-lines ; 

 rill-marks about stones and shells (Fig. 63) ; and diagonal lamination 

 (Fig. 61 e), an effect of tidal currents. Fig. 63 is drawn from a slab 

 of Medina sandstone. All these peculiarities evince that the accu- 

 mulations, while forming, were partly in the face of the waves and 

 currents, and partly exposed above the waves to the drying air or 

 sun, and to the rills running down a beach on the retreat of the tides 

 or waves. 



The structure of the Niagara limestone is often nodular or con- 

 cretionary. In Iowa and some other parts of the West, the rock 

 abounds in chert or hornstone, which is usually in layers coincident 

 with the bedding, like flint in chalk ; and the fossils are all siliceous. 

 At Lockport, N. Y., cavities in the limestone afford fine crystalliza- 

 tions of dog-tooth spar (calcite) and pearl-spar (dolomite), with gyp- 

 sum, and occasionally celestite, and still more rarely a crystal of fluor. 



The Niagara limestone (like many others) sometimes breaks ver- 

 tically with smooth columnar surfaces ; and such specimens have been 

 called Stylolites. Prof. O. C. Marsh has shown that the columns are 

 often capped by a shell ; and that this shell has, in some way, kept 

 the material beneath from the compression which the parts around 

 underwent, and hence the vertical surfaces. The shell probably 

 acted by causing an earlier hardening of the material it covered. 



Economical products. — The Ulster lead and copper mine, near 

 Redbridge, N. Y., is situated in the Shawangunk Grit : it has afforded 

 large masses of galena and copper pyrites, with blende, but is not 

 worked. The Ellenville and Shawangunk mines are others of similar 

 character in the grit. 



Mineral oil occurs in large quantities in the Niagara limestone at 

 Chicago, though not capable of being collected to advantage. Worthen 

 says, that a portion of the limestone is " completely saturated with 

 oil." 



