284 R. D. Irving—Age of the Metamorphic Rocks 
to show the association of the quartzite with the undisturbed 
Silurian rocks. 
A, A, A, Southern limit of Azoic gee crt 1,2 2 8 4,5, Masses of quartzite scattered within t the 
et wer Silurian areas. 4, Sauk y Quar zites. 5, Portland, Dodge et Quartzites. 
6, 7,8, 9,10, Granite Taasbon within a ieee Silurian area (according to * B, Northern 
Mi *higan Peninsula, (:;, Lake Superior, D, Lake Michigan. E, Illinois. —— G, Pyinn esota, 
The quartzite mass here covers an area of not more than 
three miles in an east and west direction, and much less than 
that in a north and sidan direction. On approaching from the 
east the rocks appear in the form of a low ridge, whose height 
in no case exceeds 75 feet above the general level of the coun- 
try. The approach is across a low marshy ground, and the 
ride ge though not high is thus made to stan d out somewhat con- 
spicuously. This marshy ground runs along all the western 
pret and at the northern end Siow Sew encircles one portion 
the ridge, making a marsh island of it e junction of two 
shares marsh streams is near by, a nd in times of high rand 
Island.” The highest point of the ridge, as well as the most 
peas exposures of rock, is found at this place. The ridge 
s very narrow from east to west, and descends almost imme- 
diately on the eastern side to a shallow valley. The more 
