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MR. C. BAVISOlf OK THE 



19. Note on the Movement of Scree-Mateeial. By Charles Datisoi^, 

 Esq., M. A., Mathematical Master at King Edward's High School, 

 Birmingham. (Eead Eebruary 29, 1888.) 

 (Communicated by Prof. T. G. Bonney, D.Sc, LL.D., F.R.S., F.G.S.) 

 The slope of screes being, as a rule, not much under the greatest 

 angle at which it is possible for their component material to rest, 

 it follows that a very slight force is in general required to put the 

 surface-stones in motion*. This is evident, too, from the number 

 of stones dislodged when a block falls down from above ; also from 

 the difficulty experienced by persons in trying to cross them. 

 '• Every movement," says Scoresby, describing the descent of a 

 mountain covered with loose stones in Spitzbergen, " was a work of 

 deliberation. The stones were so sharp that they cut our boots and 

 pained our feet, and so loose that they gave way almost at every 

 step, and frequently threw us backward with force against the hill. 

 "We were careful to advance abreast of each other, for any indi- 

 vidual being below us would have been in danger of being over- 

 whelmed with the stones, which we unintentionally dislodged in 

 showers " f. 



The instability of scree-material being so great, the causes of its 

 motion are consequently numerous. Many have at various times 

 been pointed out, more especiallj" in considering the origin of dif- 

 ferent accumulations of angular debris, such as the limestone- 

 breccias of Gibraltar or the stone-rivers of the Falkland Islands, 

 the main difficulty in these cases being, however, to account for the 

 transport of the material over surfaces inclined at a sm.all angle. 

 References to these well-known discussions are perhaps hardly 

 necessary, it being the object of this paper to call attention to one 

 other cause of movement which, at least in the present application 

 of it, seems to have passed unnoticed. 



While sitting near a shale-heap some time ago on a dry warm 

 summer day, I was surprised by the fall close beside me of several 

 blocks of shale, followed by a number of smaller pieces dislodged 

 by their movement. And, again, in a slate-quarry, I have noticed 

 fragments of slate on a waste-heap tumbling down and carrying 

 others along with them in their course. In neither case, so far as 

 I could see, was there any visible reason for the disturbance. All 

 around being still, the movement could only be attributed to the 

 expansion of the stones by the sun's heat during the day J. 



* Mr. Kuskin has given a useful list of the angles of screes observed by him 

 in different parts of Switzerland in his ' Modern Painters,' vol. iv. p. 317. In 

 making the experiments of the first kind afterwards described, I found that 

 when the bricks rested at angles nearly equal to the angle of friction, the 

 tremors due to carts passing at a distance of 8 or 9 yards were sufficient to 

 shake them down. It was for this reason that I afterwards made the experi- 

 ments with angles so low as 20°. 



f Arctic Regions, vol. i. p. 129. 



I Does not the fact that so many stones on the surface of screes are Just on 

 the point of slipping show that the cause of movement is not, as a rule, parox- 

 ysmal, but continuously acting and gradual in its effects ? 



