Summary . 
271 
character, except when they have been subjected to great dynamic 
action. * * * The great mass of the Devil’s Lake quartzite of Cen¬ 
tral Wisconsin exhibits perfectly, under the microscope, its frag¬ 
mental character, but along certain narrow zones slipping action 
has taken place; the grains have been elongated in a common 
direction, and it is hard to find the clastic cores if any exist. ” 
Later observations upon the Huronian quartzites made by Dr. 
Van Hise indicate that instances of dynamic metamorphism are 
not altogether uncommon among them. These, however, occur in 
comparatively narrow areas and where structures similar to 
these found in the Waterloo area are noticed. Differences in the 
composition of the rock are generally sufficient to distinguish 
them. 
A beautiful instance of the extent to which the deformation 
of quartz can be carried by pressure has been described by 
Futterer.* In his description of the quartz porphyries of Thai 
in the Thuringian Forest, he mentions the apparent stretching 
and bending of quartz grains into forms of several times their 
original length. He notes the disturbance in the molecular ar¬ 
rangement which appears in the optical behavior of the mineral, 
a bisectrix emerging in some sections as in the case of biaxial 
minerals. 
SUMMARY. 
The quartzites here described occur in a broad basin deeply 
eroded into the Lower Silurian strata in southern Wisconsin, and 
are grouped into four areas separated by considerable in¬ 
tervals of surface underlaid by sedimentary rock. From 
the correlation of observations upon the strike and in¬ 
clination of the strata, a synclinal fold is indicated having 
an east .west axis; the fold appears to have an easterly pitch 
and comes to the surface in the vicinity of the Portland out¬ 
crop. The direction of dip in the Mud Lake and Lake Mills 
areas would seem to outline the position of the synclinal. Dips 
as high as sixty degrees occur in the Portland area, and the 
rapid variation both in strike and dip even within a single ex- 
* Die Ganggranite von Grosssachsen und die Quartzporphyre von 
Thai im Thuringer Wald. Inaug. Diss. Heidelberg, 1890. 
ii—3 
