Tin Islands of the Northiwest.— Claypole. 935 
as distributing centres of the Pre-Cambrian rocks which lie scat- 
tered over the surface to the distance of 150 miles from the peak. 
They lie below the Miocene beds, and disappear under them at 
their first occurrence, and are even seen below their outliers. 
The means and manner of the distribution of the boulders has 
been a matter of some speculation. Prof. Carpenter was once in- 
clined to believe that they proved the occurrence of an Kocene 
ice-age, but has since abandoned that view. It is not easy, how- 
ever, to see how such boulders could be scattered so far over the 
country merely by the ordinary agents of erosion. 
The Miocene deposits were laid down in a great fresh-water 
lake, and in them are entombed the remains of a gigantic mam- 
malia of that era, indicating the proximity of land. During all 
this time the Black Hills stood as islands in this lake, and were 
doubtless tenanted by the animals whose remains are found in the 
strata. Miocene deposits approach within 15 miles of the Hills. 
Here the geological record of the region ends. With the ex- 
ception of a few fragmentary notes of Quaternary time made by 
the usual stream erosion and of the relics of a few Pieistocene 
mammalia that occupied the Hills, the later annals do not exist. 
The great Ice-Age has left on the Black Hills no sculptured 
hieroglyphics of its ice-chisel, such as those that have immortal- 
ized it elsewhere. No erratic boulders, no arctic drift indicate an 
invasion by the northern ice, and no striation or grooving of the 
rocks leads to the belief that the Hills were an independent cen- 
tre of glaciation. This fact is full of significance. Lying four 
hundred miles north of the southern edge of the great ice-sheet 
they were yet out of the zone of accumulation, and even beyond 
the zone of waste. The ice of the northeast and of the north- 
west expended its strength in crossing the wide prairie regions 
that separate these hills from the great glacial centres of the con- 
tinent, 
X. 
But the chief interest of the region at the present time lies in 
the presence of the tin-ore mentioned at the outset of this paper, 
The economical importance of this mineral is greater than that 
of any other that the Hills are known to contain. 
Cassiterite is usually limited to the proximity of granite rocks, 
but in the Black Hills it is yet more closely limited to the granite 
veins deseribed above as occurring in the older schists. And in 
