The Mud Lake Type. 
269 
ics identical in appearance with these have been found in each 
of the distinctive areas covered by the bowlder fans. 
The Mud Lake Type —The quartzites here exposed have a gen¬ 
eral resemblance to the rock from the Portland area, but are more 
vitreous and translucent, the quartz appearing in larger crys¬ 
tals with indistinct outlines and with smaller amounts of inter¬ 
stitial material. Minute facets of mica and streaks of reddish 
opaque iron oxide appear on fractured surfaces. The coarser 
varieties range in color from light gray through pink and blue 
tones to dark red and purple, with little apparent difference in 
structure. All of these variations in color appear in the layers 
exposed within an area of a few square rods on the surface of 
the small east ledge. The larger west ledge presents a finer 
textured rock of more uniform color, which approaches in gen¬ 
eral aspect the layers exposed in the Lake Mills ledge. 
The structure shown in thin sections is distinctive. Quartz 
individuals are of a larger size than any found elsewhere in the 
series, often appearing of several square millimeters area. 
Their outlines are very irregular, the individuals closely inter¬ 
lock and comparatively little finely granulated material is pres¬ 
ent. The larger grains show evidence of much disturbance, 
fine irregular cracks traversing the section in all directions 
(Plate VIII, Fig. 5.). Sericite inclusions are abundant in sections 
from the micaceous layers. In the same sections, however, lines 
of fluid inclusions lying in roughly parallel directions divide the 
larger areas into linear fragments and indicate the position of 
the fracture planes developed by the shearing process. The 
sericite films are found for the most part along these lines of 
inclusions and their association indicates that the secondary 
mineral was formed at the time of the crushing of the crystals, 
and that the amount of internal movement was not sufficient to 
greatly displace the fragments (Plate VIII, Fig. 6.). Besides 
these secondary inclusions small crystals of magnetite are of 
frequent occurence. Microscopically also the finer grained quartz¬ 
ite from the west ledge resembles that which occupies the Lake 
Mills area. 
The arrangement of these fluid inclusions is very similar to 
those described by Dr. Van Hise from the quartzites of the 
