Meeting of the American A.si<ociation. — Upham. 227 
tissue is of two types, that which constitutes an inner medullary cylin- 
der, and that which forms the wood proper of the stem. The latter 
surrounds the medullary structures, is radiately cleft by abundant med- 
ullary (?) rays, and gives every evidence of having been exogenously de- 
veloped. The woody tissues are constituted, as in the conifers, of what 
may be called tracheides, but are unlike conifers in that the tracheides 
are marked by the scalariform mode of wall-structure and not by bor- 
dered pits. All this is confirmatory of Binney's researches. As to what 
constitutes the true medulla, the author argued that it must have con- 
sisted of loosely constructed and early decadent parenchyma, whose in- 
dividual cells are no longer identifiable. 
n. Origin of the High Terrace Deposits of the Monongahela river. 
I. C White. These terraces along a distance of more than a hundred 
miles, extending at least from Weston, W. Va., to Geneva, Pa., are at- 
tributed to deposition in slack water, amounting to a lacustrine condi- 
tion. They are practicaly level, while both the rock shelf and the 
present rock bed of the valley have a regular descent downstream. The 
clay deposits of the upper terrace level (about 1,000 feet above the sea) 
are, in places, as much as sixty feet thick, and contain multitudes of 
perfectly preserved leaves. Most of the species are of trees and other 
plants which still occupy the same region; but at least one is now re- 
stricted to the region north of the great Laurentian lakes, thus pointing 
to the influence of glacial conditions in depressing the temperature as 
far south as West Virginia. The ice-dammed lake in which these ter- 
race deposits wei-e formed is named lake Monongahela. Its formerly 
level plane is now 1,000 to 1,100 feet above the sea. 
To account for the slack water at this level, the author now adopts 
the theory advocated by Hice and Foshay and by Chamberlin and Lev- 
erett, that the original drainage of the Monongahela basin passed north- 
ward, through the Big Beaver and Mahoning valleys, to the lake Erie 
valley. But this direction of the drainage was before the ice-sheet ex- 
tending from the north reached the watershed of the present Ohio. 
Then it was reversed, and the streams from the ice melting and from 
rains rapidly cut through the cols which separated the headwaters of 
the various branches running from the flanks of the Allegheny uplift, 
thus forming the new stream which the Ohio essentially is. The sug- 
gestion of Chamberlin is accepted, that there was a col south of Wheel- 
ing to be cut down, and it is thought to have been sufficiently high at 
first to form the barrier that dammed the water up to the level of the 
Monongahela terraces, which therefore are now so explained without 
the help of the Cincinnati ice dam, although that may have been influ- 
ential to produce terraces farther down the valley. As to the time re- 
quired for the production of the present Ohio valley, by the erosion of 
cols, it was inferred that the ice-sheet may have taken thousands of 
years in slow extension from the watershed of the Ohio to its farthest 
bounds, while the rocks below Wheeling are very soft and would have 
been rapidly worn away. 
