General Features. 
487 
Dips as high as 60° appear in the Portland and Lake Mills 
areas and the rapid variations in dip and strike even within 
a single exposure, as at the quarry ledge, are evidences of strong 
orographic movements in this area. 
The rock is throughout a quartzite. In portions of the Port¬ 
land and Mud Lake areas it becomes conglomeratic and in the 
Lake Mills area are strata of magnetite schist. Sericite is the 
most abundant constituent, after quartz, while magnetite and 
zircon are present, the former commonly, and the latter spar¬ 
ingly, as primary inclusions. Microscopic examinations of the 
sections show much deformation of the quartz. Evidence of the 
detrital character of the rock has been almost obliterated 
by the crushing and re-cementation of the quartz grains. The 
development of the abundant secondary mineral, sericite, is an 
other result of this metamorphism. 
Differences in composition and degrees of metamorphism have 
produced four tolerably distinct types of quartzite, the recogni¬ 
tion of which in the fragments composing the bowlder trains is 
of great assistance in defining their distribution. These types 
correspond in the main with the isolated areas described above 
and are designated as the Portland type, the Hubbellton type, 
the Red quartzite, and the Mud Lake type. Microscopically 
the distinctions lie in the relative coarseness .of texture, the 
amount of granulation of quartz, the relative development of 
the secondary sericite and the way in which the larger quartz 
granules are united, whether by interlocking without inter¬ 
stitial material, or by cementation with finely granulated quartz 
and sericite. While these differences are not always great 
they nevertheless suffice in most cases to identify drift 
specimens with the rock exposed in some one of the parent 
ledges. Corresponding with the microscopic differences are 
variations in the aspect of the rock which, though slight and 
difficult to describe, have been found sufficient in most cases for 
the unaided eye to distinguish the erratics from the different 
areas. 
Comparison of structural features found in these rocks with 
Huronian quartzites from the areas of their occurrence to the 
