94 R. Irving—Age of the Quarizites, Schists and 
places is much‘ less than one mile. The total lengths from east — 
to west, or rather, the exact points at which the peculiar rocks — 
which make up the ridges give place to the otcianae county — 
rock, are not as yet accurately known. These lengths, however, — 
seem to be from fifteen to twenty miles. 
e rock material of the ridges is mainly a hard dark-colored 
quartzite; with this in some places are siliceous and taleo-silice- 
ous schists, and two or three kinds of conglomerate. The dip 
of the strata, which, though iri some places obscure, is in others 
very marked—and can everywhere be determined by careitl 
observation—is uniformly toward the north. The angle varies — 
from 20°-25° in the south range, to 75°-80° in the north 
The occurrence of these bold ridges in the midst of a praine 
country, together with the marked contrast between their up 
Do they, or do they not, antedate the Potsdam period? Are 
they the results of local metamorphism on the Potsdam sand- 
The facts recorded in the present article are the results of @ 
series of visits made to the localities by the writer, during We 
months of September, October and November of this : 
(1871), and they will, I think, be seen to prove beyon al 
doubt or cavil, that the quartzites and schists antedate entirely 
the Potsdam epoeh, i.e., are either Huronian or Laurentian 10 a 
Of all of the notices mentioned, none are more than bri 
nesota, makes the first mention of the quartzite. He abe a 
ose _Dr. James G. Percival, in the report of progress 0 1 
isconsin survey for 1856, refers again to the quartzites, cally 
they result from a change on the Potsdam sandstones. Mr. 
James Hall, in his report of progress to the governor of We 
consin for 1860, gives by far the most accurate desefl ame 
have been able to find. “ He refers the quartzites unhesitatinglY 
to the Huronian—but gives no proofs whatever. His pam 
