Review of Recent Geological Literature. 207 
gate value of these now inundated areas in the eastern half of our country 
when drained and brought under cultivation, will be not less than that 
of all the present rich farming lands of Ohio, Indiana and Illinois 
together. 
The processes of formation of the delta swamps of the Mississippi valley 
from the Ohio river to the gulf of Mexico are admirably described. This 
great river is remarkable in being the only one on our continent that has 
formed a delta filling the inlet of the sea at its mouth and projecting 
beyond the general coast line. From the indentations of the shore at the 
mouths of other rivers, the most notable being the Chesapeake and Del- 
aware bays, into which the tributary s‘reams have built no delta depos- 
its above the sea level, it appears that the continent has lately been much 
elevated and more recently has undergone subsidence. 
Vegetation in many forms, as peat mosses, forests and their under 
brush, and the mangroves of the southern coast, have been very efficient 
to retard the drainage of lands having only slight slopes, thus producing 
swamps and extending their area. The surface of the Dismal swamp 
has a descent toward its borders at an average rate of about twenty inches 
per mile, and the forest is extending itself into the shallow lake Drum- 
mond in the central part of the swamp. 
Pliocene sand beds, which have yielded twenty-nine species of marine 
shells, five of them extinct, underlie the Dismal Swamp district, and are 
thinly covered by unfossiliferous Pleistocene sands. The Nansemond 
escarpment, cut by the sea in the upper sands when the district for some 
time was submerged about thirty feet more than now, extends from 
Suffolk, in Virginia, south to Albemarle sound, and forms the west border 
of the Dismal swamp. Thechanges of relative elevation thus indicated, 
and several other small oscillations preceding and following it, of which 
the region affords evidence, are regarded as not improbably referable 
chiefly to the effect of ice attraction upon the sea, and to the subtraction 
of its water to form ice-sheets, during the epochs of glaciation of the 
northern part of this continent and of Europe. 
The Penokee Iron bearing Series of Michigan and Wisconsin. By 
Rotand Durer Irvine and Cuar tes Ricuarp VAN Hisk, pp. 341-507; 
with plates xx—xlii, and five figures in the text. (Accompanying the 
Tenth Annual Report, U.S. Geol. Survey.) 
This is an abstract of a monograph by these authors, which is now in 
press and will soon be published. A belt a few miles wide and about 
eighty miles long, extending from lake Gogebic, in the upper peninsula 
of Michigan, westward to lake Numakagon, in northern Wisconsin, is 
elaborately described; and the origin and relations of its rock formations 
and of its large deposits of hematite, recently opened by extensive mining 
operations, are discussed at length. Owing tothe lamented death of the 
senior author after this work was far advanced, its preparation for pub- 
lication has been chiefly done by his associate and successor in charge 
of the Lake Superior division of the survey. 
The Penokee series, occupying a width that varies from a quarter of 
