Prof. G. H. Stone—Stones of the Salt Range. 419 
tion, as has been suggested by several of the writers on this subject. 
Here in the Rocky Mountains there are fine facilities for studying 
soilcap movement, and without exception the instances of it observed 
by me show only a limited amount of polishing, the apices of the. 
angles being rounded and the surface smoothed without being 
reduced to a plane like the facets of the Punjab specimens. The 
soileap polish is substantially like that which the facetted stones 
have received since they were facetted. But I have never found, on 
any piece of float-rock, scratching that in kind or degree resembled 
the planed and striated facets of the stones under discussion. There 
is a satisfactory reason for this. The slowness of soilcap movement 
gives time for the stones to accommodate themselves partially to 
each other’s movements. If the friction were to become great 
enough to produce such wide and deep scratches as appear on the 
planed facets of the stones in question, they would partly roll past 
each other rather than slide. It seems to be certain that the 
“olaciated’’ stones have been polished since they were scratched, 
and this fine polish could be caused by soileap movement or by a 
limited amount of water- wear, 
Third.—The formation of distinct scratches constitutes a problem 
distinct from that of the planing to an even surface. Could the 
scratches have been formed by wind-blown sand or gravel ? 
Scratches may be made, first, by a point rubbing against a surface 
and being held in the same relative position towards it; second, by 
a rolling body crushing its way into a softer or more brittle surface ; 
third, both these processes may combine to produce the scratches. 
It is evident that grinding by the use of loose powders involves the 
second and third of these processes, while planing and the use of 
grindstones involve the first, and produce scratches having an even 
and sharply-defined margin. 
Grains of sand and gravel stones when impelled by the wind or 
by moving water have a rolling as well as a sliding motion. This I 
know to be the fact from observation, though it is easy to prove that 
this must be the natural mechanical result of friction applied to one 
side of a moving solid that is surrounded by a liquid or a gas. As 
a body of irregular shape, like grains of sand and gravel stones, rolls, 
or partly rolls, partly slides upon another body, new points of the 
moving body are continually brought into contact with the stationary 
body, and since the shape is irregular, the new point of contact will 
usually be situated to one side of the original point, and the track 
of the grain will necessarily be crooked. If an irregular grain be 
impelled in a straight line, it cannot often preserve its direction in 
the same vertical plane after rebounding, for the friction will be 
applied to one side of the vertical plane in which is situated the 
centre of gravity of the body, and the grain will be thrown obliquely 
sideways. So, too, air and water are constantly being thrown into 
vortices by the inequalities of the surfaces over which they move. 
For these reasons it must seldom, if ever, happen that wind or water 
can impel sand or gravel with the steady motion required in order 
to produce long straight scratches. The larger stones often produce 
