very common on the tundra often swarm with 
Limnea, Physa, Pisidium and Valvata. The 
marl which results is in some localities so 
abundant, that at Old Fort Yukon it was col- 
lected, ground up and mixed into whitewash 
which was used on the buildings of the original 
trading post, nearly forty years ago. 
In the vicinity of the Klondike the author 
notes the hummocky appearance of the hills 
‘very muchlike magnified morainic knolls in a 
glaciated country,’ though having a consider- 
able elevation. Water worn pebbles and rem- 
nants of terraces up to nearly twelve hundred 
feet were observed by him personally. 
Notwithstanding the evidences of antiquity 
afforded by some features of the landscape, 
Professor Heilprin considers that many of the 
more pronounced features of the region are 
comparatively recent. While the placer gravels 
of the streams and benches seem to indicate 
more than one denudational phase, and the 
principal stream-valleys are wide and open, 
many of their lateral tributaries are narrow and 
V-shaped, and the former appear to have been 
modified by late stream displacements. The 
present stream-beds, even of the Yukon, are 
not the most conspicuous orographic depressions 
but have been carved out much more recently, 
and it is even suggested that the emergence of 
the land from lacustrine conditions may have 
happened ‘a few hundred years’ ago. 
The author estimates that denudation in the 
immediate valleys of the main streams is taking 
place at the rate of a millimeter a day, which, 
according to his computation, would equal ‘a 
valley trough of about a foot and a third’ ina 
single year. Allowing one hundred and twenty 
days for the period when erosion is not wholly 
prevented by congeiation, the reviewer com- 
putes that the total denudation for the year 
would amount to Jess than five inches at the 
author’s rate. Now the summer rainfall for 
the Upper Yukon is very small, less than an 
inch a month, and the surface of the ground is 
covered with a dense spongy mat of vegetation. 
There seems to be no particular reason why 
there should be any appreciable denudation, ex- 
cept in the actual beds of the streams them- 
selves. The water of all these small non- 
glacial streams is notably clear, and they carry 
SCIENCE. 
[N.S. Von. X. No. 260. 
practically no sediment at the points where 
they enter the main river. Consequently it 
seems probable that the estimate of Professor 
Heilprin requires revision, even his second one, 
in which he proposes a rate of 175 feet in five 
hundred years. For a short period, and in 
certain limited portions of its bed the Yukon is 
able to move a considerable weight of débris, 
but the gravels and sands in great part antedate 
the existence of the present river, which has 
actually cut through them at but a few points 
in its 2000 mile course. 
Professor Heilprin, in view of his limited 
opportunity for research, very properly dis- 
claims any attempt to decide upon the geolog- 
ical structure of the region. However he de- 
votes a good deal of space to an argument in 
favor of the deposit of gold in the placers, not 
from preéxisting stringer leads or veins in the 
country rock, but as a deposit, ab initio, from 
gold held in solution in water, upon or among 
the already deposited gravels. This is a con- 
tention which may properly be left to metal- 
lurgical experts to discuss, to the reviewer it 
seems unsupported by any direct evidence in 
this region. The author agrees with previous. 
observers in affirming the non-glaciated charac- 
ter of the Klondike, and the presence of com- 
paratively recent indications of volcanic ac- 
tivity. Pleistocene mammals are represented 
by fossil bones in the gold gravels as elsewhere 
in Alaska, and there is little doubt that the 
placer deposits as a whole are post-glacial and 
their material largely due to denudation by ice 
action during glacial times. 
W. H. DALL. 
The Design and Construction of Dams, including 
Masonry, Earth, Rock-Fill, and Timber Struc- 
tures; also the principal types of Movable 
Dams. By EDWARD WEGMANN, C.E. Fourth 
Edition, revised and enlarged. New York, 
John Wiley & Sons. 1899. Quarto, cloth, xii 
+ 250 pages, 97 plates. Price, $5.00. 
Many mathematicians have occupied them- 
selves with the deduction of the shape which a 
high masonry dam should have in order to 
possess both stability and economy. Such 
economic profiles are of interest and value to 
the designer, but practically each engineer de- 
