NIAGARA LIMESTONE. 351 



It is difficult to state definitely the thickness of this subdivision, 

 for, as already seen, its lower limit is not well defined, and it is equally 

 difficult to fix precisely its upper boundary. 



The greatest thickness, directly observed, was 48 feet, but this does 

 not include any of the transitional beds, and probably not all others, 

 as the section was incomplete. If we include that portion of the beds 

 of passage below and above, which is most nearly allied to this divis- 

 ion, the maximum thickness will be about 70 feet. 



This formation is very closely related to the Upper Coral beds, and 

 the distinction between the two is less marked than that between the 

 other subdivisions of this group. It will therefore be a matter of con- 

 venience to describe the Upper Coral beds, and then consider their 

 extent and local developments conjointly. 



UPPER CORAL BEDS. 



These beds directly underlie the Racine limestone at the north, and 

 are separated from them by a sharp line of division, readily distin- 

 guishable wherever observed. 



The rock is a rather thin bedded dolomite, generally of a buff color, 

 as seen in exposures, but, in its unweathered condition, often grayish or 

 bluish. It is usually subcrystalline, of fine grain, compact, and hard, 

 but occasionally earthy. It shows a tendency to split into irregular 

 rudely lenticular flakes. It contains much silicious material in the 

 form of chert, flint or silicified fossils. The chert is usually white, 

 and in the form of nodules, but graduates into dark, translucent vari- 

 eties, which pass into flint, resembling that of the chalk beds of Eng- 

 land. The carbonate of lime, that originally constituted the material 

 df the fossils, has been replaced in many cases by a whitish, chert- 

 like material, and in others by translucent and transparent forms of 

 cryptocrystalline silica, while the cavities are drusy with quartz. 

 Silicified fossils are more common than the unchanged form, and on 

 weathering, these project from the surface, giving the rock a very 

 roueh, harsh exterior. The stone is of little value for construction or 



O " 



lime. Some layers make a tolerable flag. 



. Organic remains are exceedingly abundant in this formation, among 

 which corals mostly predominate. About thirty species were collect- 

 ed, and many of these occur in great number. The state of preserva- 

 tion is often very fine, owing to silicification. The more important 

 localities are tabulated below, and a full list of species and their range 

 will be found in the general table of fossils of the Niagara group. 



