sl 
286 R. D. Irving—Age of the Metamorphic Rocks, etc. 
turbance over a small area, away altogether from any great 
system of metamorphism is inadmissible. 
2d, The occurrence close by of horizontal layers of Potsdam 
age. These layers indicate the lower portion of the upper 
member of the Potsdam group. The quartzite, if altered from 
the “Lower” sandstone, must have been made, upheaved, and 
worn down in the interval between the close of the sandstone epoch 
and the beginning of that of the limestone. But these formations 
everywhere throughout the State graduate imperceptibly into 
one another; in other words, there was no interval. That the 
uartzite cannot have been altered from the St. Peter’s sand- 
stone, as stated by Percival, is also shown by these horizontal 
ayers. A sandstone could hardly be changed to quartzite, 
whilst the beds immediately underlying it are left unaltered 
and undisturbed. 
3d, The thoroughness of the change in the rocks. This is an 
additional proof that the metamorphism must have been a part 
of some great system of changes and foldings, and not the result 
of an action restricted entirely to an area of a few square miles. 
4th, The probable uniform dip to the N.N.E. The indication 
of a uniform dip at a very high angle, together with the absence 
of any sign of an anticlinal, is direct proof that the quartzites 
are older than the undisturbed beds of the Potsdam period, 
which lie near by. The time that elapsed after the deposition 
of the Potsdam beds, must have been long enough to cover the 
time of upheaval and metamorphism, as well as the time requi- 
site to erode all traces of an anticlinal. 
I may say then confidently that these rocks are older than 
the Potsdam; that they received. their present form before the 
laying down of the Lower Silurian strata; and that we find 
in them simply another outlying island in the Potsdam seas. 
frelation to the main Azoic body, and to the other detached 
Azoic masses of the State. The accompanying outline map of 
Wisconsin serves to show these relations, A, A, A, is the 
southern line of the Azoic body; 1, 2,38,4,5, the patches of 
goatee according to Lapham’s map, lying within the Lower 
ilurian area; 6,7,8,9,10, are similarly isolated masses of 
granite or granitoid rocks. The Sauk quartzite ridges are 
marked 4; that of Portland 5. It will thus be seen that the 
last named is much the most distant from the Azoic body. _ 
Can these quartzite areas be regarded as Huronian? The kinds 
of rocks (quartzites, conglomerates, siliceous slates), the dis- ° 
tinct stratification, the no less distinct lamination, Mp le 
markings, ete. (Devil’s Lake), and the absence of granitoid rocks 
would seem to show a close similarity between the rocks of 
these isolated areas and those in northern Wisconsin and north- 
ern Michigan now regarded as Huronian. 
University of Wisconsin, Dec. 14, 1872. 
