192 



SCIENCE. 



search, any thing like the ordinary parasitic fungi. It 

 was long ago conceded by entomologists that the 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 1 a by 3.5 u 

 (.0000343 in. by .0001202 in.), made up of several 

 not very evident articulations. They rest in some 

 stages nearly or quite motionless, and in this condition 

 show a curious peculiarity of hying 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 flowering 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 ;tn investigation of the surface geology 

 of southeast Pennsylvania, the writer has determined some 

 fa<is, regarding one of the gravels, which, bearing directly 

 upon the antiquity 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- 

 < hi, it will be important to know to which of these gravels 

 they should be referred. 



The surface formations of southeast Pennsylvania may be 

 divided into five 1 lays and foui gravels. These are, begin- 

 ning with the oldest: (1) Jurasso-cretaceoui plastic clay, seen 

 at Turkey Hill, Bucks Co. ; (2) Tertiary clays of the " Bran- 



* Head before the A. A. A. S., Boston, 1880. 



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) " Branch town clay" of similar age; (5) " Glass- 

 bord 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 redgravel," 

 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 upstream. 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 tjie gravel instead of flowing upon it, as at Phila- 

 delphia. This explanation dispenses with the necessity of 

 assuming, as some geologists have clone, 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 dry land 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 

 Hooded 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- 

 ograph}' of its banks, the comparative amount of its erosian 

 and tlii' character of its materials, all point to the conclu- 

 sion that it was deposited by a great Hood of the river ; and 

 this, when taken in connection with the fact that it lies within 

 a channel cut through gravel deposited by the waters of the 

 melting glaciei indicates a past glacial and comparatively 

 recent age of the Trenton gravel. 



The important bearing of this fact upon the antiquity of 



man on tin: Delaware, which, as will appeal, depends di- 

 rectly Upon the aye 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 lock need not have been long. On (he 



