884 The American Naturalist. [October, 
the oceanic connection. The changes of level have been of two char- 
acters ; (a) the epeirogenic or continent-making movements, which 
produce broad but gentle undulations, depressing basins or raising up 
barriers, but not distorting the topographic features so as to render 
them unrecognizable, and (b) orogenic or mountain-making move- 
ments, which are most energetic over limited zones, and produce disfig- 
uring barriers. Whilst the Antillean region was sinking with gentle 
undulations, the Central American mass was slowly rising, but it was 
farther deformed by the great mountain making movements and the 
late voleanic accumulations, which have completed the separation of 
the Antillean Seas and the Pacific Ocean. 
The phenomena are extremely suggestive, and from the evidence 
brought out it appears that many problems of physical geology will 
need readjustment in the light of the changed continental condition, 
ocean currents, climate and distribution of life. The subject is important 
as a contribution to the structure of land features in their inter- 
pretation of geological history. 
J. W. SPENCER. 
The Drainage of the Great Lakes into the Mississippi 
River by way of Chicago.'—I now add another short chapter 
to the history of the Great Lakes. The highest beach south of 
Chicago is 45 feet above the lake and there are several beaches 
just above the present lake level. The divide between the lake and 
the Mississippi drainage is only eight feet abovethe lake, and this at 
a point 25 miles southwest of Chicago. The succession of beaches 
at the head of the lake has led to confusion, as there is an enormous 
lapse of time between, for the highest amongst the oldest shore lines of 
the later region from its level the lake shrinks to a plain 300 feet below, 
whilst the waters were being drained by way of the Huron Basin and the 
Ottawa River. Afterwards terrestrial deformation raised the northeast- 
ern river of the basins and turned the Huron waters into the Erie 
and Michigan basins, and for a time overflowed the Chicago divide, 
which became drained about 1500 years ago by the recession of Niagara 
Falls through Johnson Ridge. With the terrestrial deformation con- 
tinuing as in the past, it is estimated that the drainage of all the upper 
lakes may be turned into the Mississippi in about 5000 or 6000 years. 
J. W. SPENCER. 
Geological News.  GrNERAL.— Professor T. C. Bonney calls at- 
tention to the possibility that a rock of igneous origin can be so 
‘Abstract of paper read before the American Assoc. Adv. Science. 
