Pembina Region ot North Dakota — Berkey. • 145 
characteristic of continuous beds. Yellowish color is local 
and best developed where conditions are favorable to 
weathering and leaching action, while the brown facies is in 
all cases due to staining of iron oxides formed in the decay 
of iron pyrite. 
In physical characters all are moderately soft earthy 
shales. The sediments of which they are ^formed are of 
very fine grain and remarkably uniform. There is an oc- 
casional concretion of lime or iron sulphide, or a septarian 
nodule, or a selenite crystal or even a thin slab of 
lime stone, but these are confined to a few horizons and 
are insignificant in comparison to the great mass of shales 
exposed. There is occasional faulting on a very small 
scale. There is local leaching and a little chemical modi- 
fication, but the changes are slight. 
Petrographically they vary from black carbonaceous 
shales to white alum shales, and from bluish gray or yellow 
marls to bluish green or greenish black clay shales. Calcite, 
pyrite, selenite, and limonite are secondary minerals of 
most prominence in the formation as a whole. 
Chemically the beds vary greatly. Lime, iron, and car- 
bonaceous matter are the chief disturbing factors. One 
small seam of coal a fraction of an inch thick has been 
seen, but in general the carbonaceous content is not high 
in the regular shales. Lime however is important ranging 
in the various beds from a fraction of one per cent to ex- 
treme values of over 70 per cent in the form of carbonate. 
Iron is as a rule more prominent in the non-calcareous beds. 
This interchange of predominance becomes an important 
factor in fixing the possible uses to which these clays may 
be put. 
Su/>-division^ - — There are few sharp lines of distinction 
between the successive beds. Even where chemical analysis 
shows radical differences,- the formations sometimes present 
a most deceptive uniformity to the eye. Combining, how- 
ever, such physical breaks as are apparent with the chemical 
diflferences that are known, the following general succession 
is characteristic and can be identified throughout the dis- 
trict : 
E. Black and grayish brittle shales that break out in chips or in 
thin laminae like paper shales. Where carbonaceous matter is 
