172 The American Geologist. ; March, 190: 
formations, the actual distribution of such residuary deposits 
is very limited. This is no doubt largely due to the invasion 
of glacier ice which transported and worked over almost all 
of the original weathered zone. Post glacial decay has ac- 
complished little. The numerous outcrops of granite, gneisses, 
and gabbro are comparatively fresh. So complete has been 
the destruction of this earlier weathered zone that only in the 
most protected areas is there any remnant of it still to be 
found. Ina few places in the Minnesota valley, however, there 
are exposed deeply decayed granitic gneisses. Such an oc- 
currence at Redwood Falls has attracted some attention from 
time to time but has not seemed to warrant development. 
2. Common residuary clays are from the insoluble residue 
left on the surface exposures of limestone formations as a re- 
sult of continuous weathering and loss of soluble constituents. 
They are of limited areal extent for largely the same reasons 
as in the above case. In the “driftless area’’ of southeastern 
Minnesota, however, there are deposits of this type. There is 
no noteworthy brick production from clays wholly of this ori- 
gin. Deposits in this area are so intimately associated with 
loess accumulations, which are also used, that distinctions can 
be made only in individual cases. 
Il. Transported Clays. 
A. Sedimentary Formations Used as Clays. There are 
several different formations used, the most valuable being 
Ordovician and Cretaceous shales. 
1. Argillaceous Slates. These are true slates—older meta- 
morphosed shales. A plant at Thomson, thirty miles south- 
west of Duluth, is the only one that has tried this kind of 
material. The slates have to be crushed and the dry press 
process is used. Bricks made from this formation were good 
in quality and finish, but the expense and difficulty of handling 
such material seems to have discouraged the enterprise. No 
product has been reported for several years. There is no fault 
in the method of working, but the slates are very hard and do 
not lend themselves easily to economic working. 
The slates themselves are not of great extent. Cloquet, 
Thomson and Carlton mark the locality about which they 
occur. 
