NIAGARA PERIOD NIAGARA EPOCH. 



239 



potash, carbonate 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 do- 

 lomite. 



The structure of the limestone is often nodular or concretionary. 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 

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

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

 limestone sometimes breaks vertically with smooth columnar surfaces, — a pecu- 

 liarity which has been attributed to the crystallization of a foreign substance. 



II. Life. 



Corals and crinoidal remains are so abundant in the Niagara 

 limestones, and, in many places, so far make up the rock, that 

 the beds have been well called old coral reefs. The variety of life 

 about the reefs was very great. We find, besides the flowering 

 Corals and the slender-rayed Crinoids, great numbers of Brachio- 

 pods, some Conchifers, large Orthocerata, and many new kinds of 

 Trilobites. There is no evidence of any fishes, and none of life 

 over the land or in fresh waters. 



Figs. 383-388 represent some of the Corals. Fig. 383, one of 

 the Cup-corals; fig. 385, Halysites catenulata, projecting above the 

 limestone in which it is imbedded, — it is often called Chain-coral, 

 from the appearance in a transverse section ; fig. 384, Favosites Nia- 



Figs. 383-388. 



85 



Corals.— Fig. 383, Chonophyllum Niagarense ; 384, a, Favosites Niagarensis ; 385, Halysitea 

 catenulata; 386, 387, Heliolites spinipora; 388, Stromatopora concentrica. 



garensis, a Coral of a columnar structure, with horizontal partitions 

 in the cells. Three out of the many fine Crinoids are represented 



