192 
SCIENCE. 
search, any thing like the ordinary parasitic fungi. It 
was long ago conceded by entomologists that tire dis- 
ease did not arise from the depredations of insects. 
I am now able to confidently assert that this devas- 
tating disease of the peach is caused by Bacteria ! 
These minute, moving, living things are found in 
great numbers within the cells of the diseased tree. 
They are apparently specifically different from those 
of the pear tree, being comparatively much more slen- 
der. What I take to be the typical form — all 
vary considerably — is very nearly i /i by 3.5 /i 
(.0000343 in. by .0001202 in.), made up of several 
not very evident articulations. They rest in some 
stages nearly or cpiite motionless, and in this condition 
show a curious peculiarity of ly ing in ranks, side by 
side. In other periods of development they move in 
an unsteady, undulating manner with considerable 
rapidity ; they turn, twist and tumble on their sides, 
on end, now drifting with the current, now swarming 
in an inextricable maze in the field of a first-class one- 
tenth objective. 
As the Bacteria increase the starch grains, stored 
by the tree for its own nourishment disappear, and I 
doubt not further investigation will prove that, as in 
the blight of the pear and apple, butyric fermentation 
takes place. The diseased tree probably suffers in 
other ways from the presence of these minute para- 
sites, but we may say with truth that it really starves 
to death. Its food, gathered from the earth and air, 
assimilated by the leaves and stored for immediate or 
future use, is ruthlessly seized upon and destroyed. 
No doubt this takes place at all times of the year, 
when the temperature of the surrounding air is con- 
siderably above the freezing point ; but the Bacteria 
are probably most active in the summer time. 
Judging from my experiments upon the pear tree, 
the destroyers only gain entrance to the tissues of the 
tree through wounds in the epidermis or bark ; but it 
is possible that at the time of Powering they penetrate 
by way of the stigma, which is not protected by an 
impervious coating. 
The cellulose tissue of the tree is not destroyed, 
and it is still a puzzle how the Bacteria, minute as 
they are, pass from cell to cell. As in the pear, it is 
probably a very slow process, and is not connected 
with the circulation of fluids in the tissues. 
The discovery of Bacteria as the cause of disease 
in plants may prove a notable contribution to the 
“germ theory” of disease in animals. 
THE ANTIQUITY OF MAN IN EASTERN 
AMERICA, GEOLOGICALLY CONSIDERED* 
By Henry Carvill Lewis, A. M. 
In the course of an investigation of the surface geology 
of southeast Pennsylvania, the writer has determined some 
facts, regarding one of the gravels, which, bearing directly 
upon the antiquit)' of man in America, become of interest. 
In former papers the writer has shown that the gravels of 
the Delaware Valley belong to several distinct ages; and if 
therefore at any place the remains of man are shown to oc- 
cur, it will be important to know to which of these gravels 
they should be referred. 
The surface formations of southeast Pennsylvania maybe 
divided into five clays and four gravels. These are, begin- 
ning with the oldest: (1) Jurasso-cretaceous plastic clay, seen 
at Turkey Hill, Bucks Co.; (2) Tertiary clays of the “ Bran- 
don Period'' associated with the iron ore, kaolin and lignite 
of the Montgomery County Valley ; (3) “ Bryn Mawr gravel," 
often found at elevations of 400 ft., characterized by the 
presence of an iron conglomerate and of pebbles of Pots- 
dam, but never of Triassic rocks, and conjectured to be late 
Tertiary; (4) “ Branchtown clay " of similar age; (5) “ Glass- 
boro' gravel," of latest Pleiocene age, found also on the 
watershed in New Jersey, between the Atlantic and the 
Delaware, and known by its pebbles of Niagara limestone 
and of other fossiliferous rocks; (6)“ Philadelphia red gravel,” 
of Champlain age, which contains numerous boulders of all 
materials, fragments of Triassic rocks, etc., which shows 
flow-and-plunge structures and wave action on a large scale, 
which rests on a decomposed gneiss, and which is confined 
to the river valley; (7) Philadelphia brick clay, which, with 
its boulders, rests upon the last, aud like it, appears to have 
been deposited by the waters of the melting northern gla- 
cier; (8) “ Trenton gravel,” a sandy river gravel forming the 
bed of the Delaware; (9) the modern alluvial mud now form- 
ing in the tidewater swamps. 
Of these formations, one of the least conspicuous at 
Philadelphia is that now called the Trenton gravel. It is a 
true river gravel, rising here but a few feet above the water, 
and forming a quicksand when below water level. It is of 
gray color, and contains pebbles composed entirely of the 
rocks which form the upper valley of the river. Unlike 
older gravels, it has very few quartz pebbles, and its peb- 
bles are generally flat. In the middle of the river at Phila- 
delphia it is 100 ft. deep. On tracing this gravel up the 
Delaware it is found to rise higher above the river and to 
extend farther back from it as we proceed up stream. .Thus, 
at Bristol it extends two miles back from the river, and is 
bounded by a well-marked hill, upon which rest the older 
gravels. At Trenton, the limit of tidewater, the narrow up- 
land portion of the valley begins; and from there up this 
gravel is shallow, and confined to the river bed. The 
oceanic gravels trend across New Jersey, and are no more 
seen. Two surface formations alone remain — the river 
gravel of past glacial age, and the brick clay, with its bould- 
ers, of champlain age. The first lies within the last, and 
both can be traced up to the great terminal moraine near 
Belvidere. It is to be especially noted that the Trenton 
gravel is newer than a drift of champlain age. It is in this 
Trenton gravel, and in this gravel only, that traces of man 
are found. 
The Trenton gravel at the locality which gives it its name, 
is remarkably well exposed. Trenton is at the point where 
a long narrow valley with continuous downward slope opens 
out into a wide alluvial plain, and where the rocky floor of 
the river suddenly descends below ocean level. It is here 
that the bulk of a gravel, swept down the upper valley, 
would, on meeting tidewater, stop in its course, and with 
its boulders be heaped up in a mass, immediately afterward 
to be cut through by the river. It was thus that a cliff of 
gravel 50 ft. high was here formed, the river having cut 
through the gravel instead of flowing upon it, as at Phila- 
delphia. This explanation dispenses with the necessity of 
assuming, as some geologists have done, the submergence 
of the land by the ocean at the time of the deposition of the 
gravel. That Southern N. J. was at that time dryland is 
shown by the fact that this gravel at Trenton extends inland 
a few miles only, and having filled up a bar in the ancient 
flooded river, is bounded by hills of the older gravel which 
forms Southern N. J. 
There are many facts indicating that the Trenton gravel is 
a true river gravel and not a glacial moraine, which are de- 
tailed in the present paper. The absence of glacial marks 
on the rocks, the stratified character of the gravel, the top- 
ography of its banks, the comparative amount of its erosian 
and the character of its materials, all point to the conclu- 
sion that it was deposited by a great flood of the river ; and 
this, when taken in connection with the fact that it lies within 
a channel cut through grav'd deposited by the waters of the 
melting glacier indicates a past glacial and comparatively 
recent age of the Trenton gravel. 
The important bearing of this fact upon the antiquity of 
man on the Delaware, which, as will appear, depends di- 
rectly upon the age of this gravel, is here apparent. Calcu- 
lations based upon the erosive power of running water show 
that the time necessary for the river to cut through this 
gravel down to the rock need not have been long. On the 
* Read before the A. A. A. S., Boston, 1880. 
