356 GEOLOGICAL APPEARANCES 



taken by the people for the petrified froth of the river, 

 It seems to be formed in consequence of a decomposi- 

 tion of the pyrites imbedded in some parts of the rock. 

 The sulphuric acid to which this process gives rise* ex- 

 pels the carbonic acid, and unites with the limestone by 

 Virtue of a more powerful attraction. Thus the com- 

 mon limestone is converted into plaster of paris ; or in 

 chemical language, the carbonate of lime is changed in- 

 to a sulphate. The two compounds very commonly 

 exist together, the limestone and gypsum cohering and 

 making parts of one mineral mass. In some rills where 

 the brimstone appears not to have been combined with 

 oxygen, it oozes out with the water, and discolours the 

 rocks. Thus native sulphur and calcarious sulphurets, 

 may be enumerated among the natural products of Nia- 

 gara. 



The siliceous ingredients in the rocks hereabout con- 

 sist mostly of quartz and flint. The quartz is sometimes 

 mingled with the calcarious carbonate in such quantity 

 as to give sparks with steel ; forming a sort of siliceous 

 limestone. In other cases it exists in veins or streaks al- 

 most unmixed. And lastly it bespangles the surface with 

 elegant crystals, hard enough to scratch glass. The flint 

 at the falls is whitish ; but near the outlet of lake Erie it 

 is blackish. In both places it is distinctly bedded in the 

 limestone; and their quantity is relatively small, particu- 

 larly at the former place. At the latter, the colour of 

 the flint has concurred with that of the calcarious 

 strata in which it is immerged, to obtain for the spot the 

 name of Black rock. This stone breaks with the conca- 

 vo-convex fracture ; and answers very well to furnish 

 fire-stones for muskets. It does not seem to be chemi- 

 cally incorporated with the limestone ; but to be laid in 

 it as pebbles are scattered through breccias. It puts me 



