192 On the Silurian System of the Lake Superior Region, 
stances, and their persistence over so great an area, where phys- 
ical characters have in a great degree failed, only serve to demon- 
strate the value of these means of studying and identifying the 
stratified deposits. 
I have, also, had an opportunity of tracing this group, at inter- 
vals, across the country between Lake Michigan and the Missis- 
sippi river, and of recognizing it by its fossils, particularly its co- 
rals, even beyond that point, to the southwest. It exists in 
Ohio, Indiana, Kentucky, and Tennessee, and may be recognized, 
not only by its lithological characters, but by its numerous fossils, 
identical with those first described in the New York Geological 
Reports, as occurring in this group. Among these, we have, in 
addition to the corals, which are the more common fossils, several 
yo- 
characteristic fossils. His observations will be found incorpora- 
ted in the subsequent pages of this-report. 
Thickness of the Niagara and Clinton Groups.—As already 
remarked in the commencement of this chapter, the lowest beds 
are seen at the lake-level on the eastern side of Drnmmond’s ist 
mond’s island. The entire height of the cliffs does not exceed 
two hundred and fifty feet, and we have nowhere evidence of the 
existence of superior beds of more than one hundred feet in thick- 
ness belonging to these groups. The entire thickness, therefore, 
a fa Qa oes 
* Geological Survey of Canada, Report of Progress, 1847-48. 
