4-28 



THE POPULAR SCIENCE MONTHLY. 



represent rivers or inlets of the Tertiary sea ; 

 that they are the beds of extinct lakes ; and 

 that they represent the beds of Quaternary 

 rivers. Mr. R. T. Hill, who has made a 

 geological examination of the region, finds 

 all these theories wrong. lie sees in the 

 ground on which the timbers grow, the de- 

 tritus of arenaceous strata which occupy 

 well-defined horizons in the geologic scries, 

 and which have been exposed by the denu- 

 dation of the overlying strata. The timber 

 confines itself to these arenaceous belts be- 

 cause they afibrd a suitable matrix for the 

 penetration of the roots of trees and a 

 constant reservoir for moisture ; while " the 

 barrenness of the prairies, so far as forest 

 growth is concerned, is owing to the ab- 

 sence of the requisite structural conditions 

 for preservation of moisture as well as to 

 the excess of carbonate of lime in their 

 soils." 



Injnrios canscd by Parasitic Fangi. — 



The injuries which parasitic fungi produce 

 upon their host plants, were described in 

 the American Association by A. B. Seymour. 

 Parasites take away the nutriment of the 

 plants, killing or continually absorbing the 

 food-supply of individual cells, and injuring 

 cell-walls. They impair the power of assimi- 

 lation, weaken the physiological power, caus- 

 ing the formation of spots and of black 

 molds to obstruct the passage of light. 

 Some fungi cause a change of position in 

 their host, and less favorable exposure. 

 They provoke abnormal acceleration or re- 

 tardation of growth, with resultant distor- 

 tion and impaired vitality. Any part of 

 the plant may be affected. In many grasses 

 the entire inflorescence is destroyed. De- 

 cay is produced in ripe fruits, and valu- 

 able plants become infected with disease 

 from less valuable ones. The extent of the 

 injury attributable to any one cause is hard 

 to determine, because several causes act 

 together. 



Dccpptlve Sensations. — The evidence of 

 our senses is correct in nearly all cases in 

 which two of them are called into play, so 

 that the testimony of one is checked by that 

 of the other. But when we have to rely 

 upon one sense alone, we are sometimes 

 liable to curious deceptions. This is the 



case with some of the feelings of touch. In 

 hip-joint disease the pain is often referred 

 to the knee, while it is really in the hip. 

 This is because the nerve which conveys 

 sensation from the knee, also sends a branch 

 to the hip-joint. The experiences of those 

 who have lost a limb are familiar. For 

 some time afterward they feel sensations 

 and pains of all sorts in the member that is 

 gone, BO that they can hardly convince 

 themselves that it is not still there, itching 

 or aching or smarting. This is because 

 the nerve which used to convey feeling to 

 the lost extremity is affected by some tem- 

 porary accident. The feeling, which may 

 be real, as to the trunk that remains, is 

 still, by force of habit, referred to the ex- 

 tremity whence it used to come. Another 

 experiment in deceptive sensations may be 

 made by crossing the second finger over 

 the first, and then placing a marble between 

 the tips of the fingers, when it will be almost 

 impossible to convince one's self that there 

 are not two marbles. This is because two 

 points in the fingers are touched simulta- 

 neously, which in the ordinary position 

 could only be touched at the same time by 

 two marbles. Acting upon its previous 

 knowledge, the brain says that there are 

 two. 



Relative Mortality of Social Classes, — 



Mr. Xoel A. Humphreys, in a paper on the 

 relative mortality among the different classes 

 in society, after citing the general evidence 

 of the British life-tables that the mean 

 duration of life has perceptibly and steadily 

 increased in recent years, shows that this 

 factor is mainly controlled by the rate of 

 mortality in Childhood ; and the expectation 

 of life is regarded in the life-tables as great- 

 er at ten years of age than it is at birth. 

 Subsequently to childhood, the greater vital- 

 ity of the upper and ndddle classes, com- 

 pared with that of the general population, 

 is only somewhat less marked than it is at 

 under five years of age. The statistics of 

 mortality, according to occupation, show a 

 great difference in favor of quiet pursuits, 

 and, among workingmen, of those engaged 

 in the country as against those occupied in 

 cities. Many of these differences are doubt- 

 less due more to the influences and risks of 

 occupation than to the mere influence of 



