560 Notices of Memoirs—J. Lomas— 
a grain is great as compared with the width the rolling may be 
confined to one axis of revolution and a cylindrical form results. 
Cylindrical grains of this nature are not uncommon in desert sands, 
and they are found in the dunes round our coasts. The sands rolling 
up a slope often form the most exquisite ripples, giving the appearance 
of tiny dunes riding on the larger ones. Perfect spheres do not, as 
a rule, result from the attrition of very small grains, and we seldom 
find ‘Millet seed’ sands with a diameter of less than “5mm. Not 
only does abrasion take place by the striking of one grain against 
another, but the battering of grains against the solid rocks or against 
loose stones lying on the surface of the desert results in some 
characteristic effects. The effects produced on the rocks will vary 
not only with their hardness and texture, but also with the angle at 
which the impact takes place. 
A glass tumbler dropped by a tourist in the desert was fone after 
a time to be frosted and opaque on those parts exposed above the 
sand. It exactly resembled the sand-blast labels on our reagent 
bottles and the designs frosted on glass intended for ornamental 
purposes. 
Blocks of obsidian found in the deserts of Iceland likewise 
become frosted on their exposed surfaces. Granite on the other 
hand takes a beautiful polish. If the rocks contain minerals of 
varying hardness the softer constituents tend to form hollows, while 
the harder materials stand out and are sometimes completely dis- 
engaged from the mass. In this way the fossils in the nummulitic 
limestone of which the Great Pyramid is built stand out from the 
surface, and large numbers of loose nummulites can be found in the 
sands surrounding the base of the pyramid. So perfectly and so 
intimately does the sand pick out the parts of superior hardness, or 
of looser texture, that pieces of silicified wood can be found in the 
neighbourhood of Cairo with the vascular fibres standing out from the 
less compact parenchymatous tissue. 
Mr. W. D. Brown! recently described before the Liverpool 
Geological Society some most interesting experiments which he 
performed with artificial sand blast on various rocks. He showed 
that a blast with a pressure of 45 pounds to the square inch, acting 
perpendicularly on sandstone, drilled a cylindrical hole and removed 
435 grains in 5 minutes, whereas the same blast acting on the same 
piece of rock at an angle of 45 degrees produced an even plane surface 
and only removed 400 grains in 10 minutes. A piece of granite with 
an oblique blast lost 200 grains in 5 minutes, while limestone lost 
$23 grains in the same time and under similar conditions. All the 
stones subjected to the experiments possessed rounded surfaces before 
being acted upon, but the sandstone and limestone were reduced to 
plane surfaces while the granite showed differential action, the quartz 
standing out from the softer constituents. 
The production of a plane surface by oblique sandblasting may be 
compared with the action of a file drawn over a curved surface. 
Being rigid, the file moves in a straight line, it does not accommodate 
1 Proc, Liverpool Geol. Soc., vol. x, p. 130. 
