60 The American Geologist. jan. isoo. 
of the primordial formations in Minnesota and the Northwest with 
those of the northeastern states. Professor Winchell denominates the 
lower three Northwestern divisions of this group in ascending or- 
der the Taconic, Potsdam, and Saint Croix series, referring the 
Keweenawan series of Irving to the Potsdam epoch. At the base of 
the Tacoaic a wide-spread unconformity is recognized, separating the 
Keewatin series, the uppermost of the Archean, from the Animike 
(Taconic) formation. 
The Rivers and Valleys of Pennsylvania. By "William Morris Davis. 
pp. 71. (Lecture delivered before the National Geographic Society 
at Washington, Feb. 8, 1889, and published in the National Geographic 
Magazine, vol. i. No. 3). The investigation presented in this essay 
was attempted with the hope of unfolding a teachable sequence of 
facts that would serve to relieve the usual routine of statistical and 
descriptive geography; but the author finds, after thorough study 
and analysis of the well determined geologic and geographic features 
of Pennsylvania, that the history of the Susquehanna, the Juniata, or 
the Schuylkill is too involved with complex changes, if not enshrouded 
in mystery to become intelligible to any but advanced students. To 
such the essay will be found very suggestive, opening a new field of 
geologic observation and induction. The single course of an ancient 
stream is now broken into several independent parts, and conversely 
the present rivers are often made up of parts that were formerly sep- 
arated by watersheds. For example, the Juniata of to-day comprises 
headwaters acquired from Ohio streams, and the lake in which the 
river once gathered its upper branches has become a mountain-top, so 
that the streams now flow around the margin of the lake, not across 
its basin. 
Preliminary to the special discussion of the development of 
the rivers of Pennsylvania, the author sets forth the gen- 
eral history which a river would- pass through in its cycle 
of youth, adolescence, maturity and old age, on the sup- 
position that a continental area were uplifted from the ocean 
and were then allowed to remain undisturbed through this period. 
But it may be doubted whether so long repose has ever been granted to 
any river basin ; and manifold changes in the course and character of 
streams have been caused by movements of elevation, depression, and 
mountain-building. 
Professor Davis finds evidence that the Appalachian mountain sys- 
tem as it was originally upheaved in the Permian era has been greatly 
reduced and indeed finally worn away, while the ridges of to-day are 
merely the relief left by the etching of Tertiary valleys in a Cretaceous 
base-leveled lowland. He therefore concludes with Powell, that 
"mountains can not remain long as mountains; they are ephemeral 
topographic forms." 
The most conspicuous proof of differential elevation or subsidence 
during the Tertiary era are the wind-gaps in the long mountain ridges? 
