Tin Islands of the Northiwest.— Claypole. 231 
strictly logical to write ‘‘Pre-Potsdam” because we can only prove 
that they antedate that era. But the total absence of fossils and 
their structure and immense thickness point strongly to a Pre- 
Cambrian date. Regarding the last character Prof. Newton 
writes: 
“The whole thickness of vertical rock with a width of about 
twenty-five miles, is believed to retain its original relation of 
parts.” So great a thickness of even Pre-Cambrian strata is 
scarcely credible. 
Immense beds of conglomerates are a featureof the slates, and 
are well displayed in many places, as for example, near Lead City 
on the railway. The pebbles consist chiefly of glassy quartz and 
quartzite, both of which are found in the older schists, but the 
conformability of the two series forbids our ascribing them to 
that source, and compels us to seek some other and more distant 
origin. The pebbles of quartzite are elongated, says Prof. 
Crosby, but those of quartz are not. 
Prof. Crosby has also called attention to a vast sheet of diorite 
(plagioclase felspar and hornblende), sometimes auriferous, which 
passes through the entire length of the easternor slaty series, and 
of which abundant fragments may be seen on the eastern slopes, 
and also to massive but very siliceous beds of hematite like those 
of the lake Superior region. He has further demonstrated from 
the presence of limestone pebbles in the Potsdam, that a bed of 
this material must be covered up somewhere in the Hills. This 
discovery may some day throw light on the age of the slates, some 
of which may, though such a supposition is searcely possible, be 
of lower Cambrian date. 
els 
After the deposition of the series above described in the sea of 
Pre-Cambrian age, an elevation of the region took place whereby 
the schists and slates were bent and folded at a very high angle 
and the Black Hills of Dakota were born. If we must regard 
the whole series as one unduplicated mass, they form a mono- 
clinal ridge with strong easterly dip. This movement alone indi- 
cates a long time, but the enormous erosion which the strata 
suffered before the Potsdam sandstone was laid down upon their 
basset edges forbids any doubt on this point. The interval rep- 
resented by this gap extends from the date of the latest slate to 
that of the Upper Cambrian, and may include the whole of Lower 
