NOVEMBER 12, 1897. 
further finds were made at this point. If 
there is no mistake about this occurrence, 
there must have been diamonds in this 
region long before the intrusion of any 
known mass of kimberlite. 
In the Orange Free State there are a 
number of localities at which the diamond 
has been found, although Jagersfontein is 
the only one which has yielded this gem in 
important quantities. The most northerly 
locality in the Free State of which I have 
heard is at Driekopjes, in the Kroonstad 
district. This district is bounded on the 
north by the Vaal River and it lies just 
south of Potchefstroom district, in the 
Transvaal. It is to be hoped that some 
geologist may eventually visit all the known 
diamond-bearing localities in South Africa 
and give the world the benefit of a compar- 
ative study. 
GuorGceE F. BEcKkEr. 
WASHINGTON, October 31, 1897. 
CURRENT NOTES ON PHYSIOGRAPHY. 
THE GREAT LAKES. 
GiLBER?’s discussion of ‘ Modification of 
the Great Lakes by Earth Movement,’ 
presented to the Detroit meeting of the 
American Association, is published in the 
September number of the National Geo- 
graphic Magazine (VIII., 1897, 233-247). 
It is truly astonishing that in the dozen 
years since the tilting of the ancient lake 
shore lines was recognized, and in our brief 
half century of accurate lake levellings, 
quantitative results as definite as those here 
announced should haye been reached. A 
change of level of 0.42 foot per 100 miles 
per century in a direction about 8. 27° W. 
seems to be assured. A line at right angles 
to this direction, drawn through the outlet 
of a lake, would have no change of level. 
All places on the lake shore northeast of 
such a line, or isobase, would emerge from 
the lake waters ; all places to the southwest 
would be slowly submerged. Ontario lies 
SCIENCE. 
727 
altogether southwest of the isobase of its 
outlet ; and, hence, the water must be en- 
croaching on all its shores; the estimated 
rise at Hamilton being six inches a century. 
Erie is similarly situated, and the rise at 
Toledo is placed at eight or nine inches per 
century. The outlet isobase of Huron- 
Michigan leaves Huron altogether on the 
northeast, and crosses Michigan near its 
middle; the water surface must, therefore, 
be lowered ten inches a century on the 
northeast side of Georgian Bay, and six 
inches at Mackinac ; while it must rise five 
or six inches at Milwaukee, and nine or ten 
at Chicago. ‘‘Chicago has already lifted 
itself several feet to secure better drainage, 
and the time will surely come when other 
measures of protection are imperatively 
demanded.” In 500 or 600 years, high 
stages of the lake will discharge at Chicago 
by the ancient outlet of glacial Lake Michi- 
gan. In 1,000 years the discharge will oc- 
cur at ordinary lake stages, and after 1,500 
years it will be continuous. In about 
2,000 years the discharge from Lake Michi- 
gan-Huron-Hrie * * * will be equally- 
divided between the western outlet at Chi- 
cago and the eastern at Buffalo. In 2,500 
years the Niagara River will have become 
an intermittent stream, and in 3,000 years 
all its water will have been diverted to the 
Chicago outlet, the Illinois River, the Mis+ 
sissippi River and the Gulf of Mexico.” 
THE LAVA PLATEAU OF SOUTHEASTERN WASH- 
INGTON. 
Tue lava plateau of the Columbia River 
basin, already described by Russell (Bull. 
108, U.S. G. 8S.) a few years ago, now re- 
ceives further attention from the same 
author (Irrigation Papers, No. 4, U.S. G. 
S.). <A broad flat dome, uplifted 2,000 feet 
over the surrounding country and well dis- 
sected, forms the Blue Mountains; so well 
clothed with rock waste that one is aston- 
ished to learn that they are composed of 
