Tin Islands of the Northwest.— Claypole. 229 
could buy the bronze after it had been made, but could not make 
it, and this, there is abundant reason to believe, was done. 
Even after the introduction of steel had destroyed the value of 
bronze weapons, so many uses for tin remained that the old Corn- 
ish mines were never entirely closed, though in the time of the 
English Plantagenet kings the royalty had dwindled down almost 
to nothing. 
jt 
Rising like islands from the vast prairies of the Northwest are 
the Black Hills of Dakota. They break through the monotonous 
plain that extends from Chicago to the foot of the Rocky mount- 
ains. Their steeply inclined axial strata are in strong contrast 
with the flat beds of the Cretaceous and Tertiary eras around 
them. To these, as to the new Cassiterides, much attention has 
been directed for some years past, in the hope that the future tin 
supply for the markets of North America will be drawn here- 
from. 
To the geologist these hills are equally interesting on account 
of the problems which they suggest, and the facts which they re- 
veal concerning the evolution of the Northwest. 
Prof. N. H. Winchell and Mr. Henry Newton were the earliest 
explorers in this field, and entered on its investigation while it was 
still in a wild state, and occupied by hostile tribes of Indians. 
To their work the present writer, and all who have followed 
them, are deeply indebted for facts and data. The writer’s own 
observations have been made mostly in the southern tin-district, 
and in some parts of the east and north of the Black Hills. 
ELE: 
The core or medial axis of the Hills consists of a ridge of 
schists and slates dipping at a high angle to the east, and often 
nearly vertical. Of these the schists are the older and underlie 
the slates. They are very micaceous, and often so full of garnets 
as to appear red, and have been likened by Profs. Blake and 
Crosby to the ‘‘Montalban” series of New England. These 
schists are heavily charged with lenticular sheets of a very coarse 
granite composed of quartz, albite and mica, which have been 
described by some as intrusive. But the evidence rather justifies 
the belief that they are really veins of segregation, as they uni- 
formly agree with the schists in strike and dip. These veins are 
