234 The American Geologist. October, 1894 
pears to be an ancient peneplain, suggesting, as before noted, 
a confirmation of the hypothesis of meteoric denudation, fol- 
lowed by warping of the earth's surface. 
Of the still greater unconformities in the Algonkian, the 
vast period anterior to the Cambrian and succeeding the now 
reconstructed Archean, nothing can be said. 
Comparison <>k Baseleveling with Gxaciation and 
Submergence. 
It remains to compare the action of the conjoined condi- 
tions which permit the formation of a peneplain with the 
effects produced on organic life by glacial periods on the one 
hand and by extensive oscillations of the land with reference 
to the sea, on the other. 
Croll* and Darwinf have amply considered the effects of 
glacial periods. Ice-sheets are potent in compelling rapid 
migrations, the crowding of indigenous and immigrant spe- 
cies within narrow limits in the extraglacial territory, and in 
causing the annihilation of the forms which cannot escape. 
When the ice disappears, an unoccupied and mostl}^ deforested 
field is open for all comers. 
The effects of submergence and elevation of the land have 
been elaborated by Sir Charles Lyell in his "Principles of Ge- 
ology." Submergence drowns out the land forms, or causes 
them to migrate: and baseleveling by marine erosion accom- 
plishes the same result, so far as it goes. 
Baseleveling by stream erosion differs from both submerg- 
ence and glaciation in keeping the field open for the occupa- 
tion by organisms and for those changes which are promoted 
by the ordinary adjustments between species. It induces 
change through the tendency to lower and broaden the area 
of lands. As this baseleveling advances in a region, the like- 
lihood of submergence increases, since, the coast being lower, 
a less depression will cause the sea to flow far inland; it de- 
creases the chance of glaciation, for, the land being lower, the 
surface is farther removed from the snow line. 
Periods of baseleveling are characterized b} T relative stabil- 
*()n Geological Time, Part III. Inquiry into the effects of Icebergs, 
Interglacial Periods, etc, Philosophical Magazine, Nov., 1808. 
(< hrigin of Species, cli. xii. 
