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22 THE MAGIC OF JEWELS AND CHARMS
the
waves will indeed modify the form, but along the lines of that already
produced by the earlier agencies. Broadly stated, those that were round
or oval would generally remain so, rectangular fragments might have
their angles worn away and become elliptical, while flat fragments
would not exhibit any notable change in shape.37
When
a group of pebbles have been long exposed to attrition by the waters
of a powerful stream, especially where the current is intermittent, and
where a large quantity of sand has been worked or blown into the stream
by freshet or wind storm, they may become rounded by the erosive action
of the water or by the abrasive power of the sand, as well as by the
attrition consequent upon their sharp contact with one another. This
is exemplified in the case of boulders in a river bed, it having been
noted in certain streams on the Navajo Reservation that while the
upstream sides of the boulders were polished and rounded, and even
sometimes faceted and etched, but little change was observable on the
downstream sides. This has been tested experimentally, holes an inch in
depth having been drilled in opposite sides of sandstone boulders, and
on examination five years later in five different localities where this
had been done, the deepest hole remaining on the upstream sides
measured but four-tenths of an inch, while in one locality the holes
had entirely disappeared, and yet so trifling was the effect of the
water on the downstream side that a blue-pencil mark had not been
washed away. Of course, the erosion of quartzite and limestone boulders
tested in this way proved to be a much slower process, amounting to
less than one-hundredth of an inch annually. Another important
consideration in the shaping of pebbles by river-water is the swiftness
of the current,
* See Herbert E. Gregory, " Note on the Shape of Pebbles," In The American Journal of Science, vol. xxxix, pp. 300-304; March, 1915; also for two succeeding paragraphs.
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