MINERALS AND GEOLOGY OF CANADA. 211 
the Niagara beds proper. Thus defined, the formation consists at its 
lower part of about twenty feet of dark-grey limestone (in part dolo- 
mitic, and in which the well-known Pentamerus oblongus, fig. 213, 
first appears), followed by a considerable thickness of dark, more or 
less bituminous, thin-bedded limestones or calcareous shales, which in 
their turn are overlaid by dark, thick-bedded limestones, also of a 
bituminous character. These relations are shewn in the sections, 
fig. 210: beds 4, 5, and 6. At the Falls of Niagara, the calcareous 
shales make up a thickness of about 80 feet, and the thick-bedded 
strata which succeed, and over which the cataract breaks, exhibit 
about the same amount ; but in adjoining localities it attains a thick- 
ness of 165 feet. Thin bands of gypsum occur in both the shales 
and limestones; and the latter contain, in various places, small cavi- 
ties and fissures filled with crystals of cale spar, pearl spar or dolomite, 
gypsum, blende, galena, &e. They often enclose, also, peculiar casts 
of somewhat doubtful origin. The general form of these is shewn 
in figure 214. Casts of this kind occur not only in the present 
formation, but likewise occasionally in the Tren- 
ton limestones, and in the strata of the Onon- 
daga and various other groups. They are 
generally known as crystallites or epsomites, 
and have probably been formed by the infiltra- 
tion of carbonate of lime into spaces previously 
occupied by crystalline masses of sulphate of 
magnesia or soda, or of some other soluble 
mineral. Many of the Niagara beds are ex- 
ceedingly rich in fossils. Some of the more 
Fig. 214. characteristic of these (in addition to the Penta- 
merus oblongus depicted above) are shewn in the following figures :— 
Fig. 215 —Vavosites Goth- Fig. 216.—Halysites catenulatus 
landica (Goldfuss). (Linnzeus). 
