f rocks to choose from, that the difficulty of making the best
selection is great, and this difficulty is constantly increasing with
the rapidly growing facilities of transportation and the increased range
of choice which this permits. On account of their desirable road
properties some rocks are now shipped several hundred miles for use.

There are but two ways in which the value of a rock as a road material
can be accurately determined. One way, and beyond all doubt the surest,
is to build sample roads of all the rocks available in a locality, to
measure the traffic and wear to which they are subjected, and keep an
accurate account of the cost both of construction and annual repairs for
each. By this method actual results are obtained, but it has grave and
obvious disadvantages. It is very costly (especially so when the results
are negative), and it requires so great a lapse of time before results
are obtained that it cannot be considered a practical method when
macadam roads are first being built in a locality. Further than this,
results thus obtained are not applicable to other roads and materials.
Such a method, while excellent in its results, can only be adopted by
communities which can afford the necessary time and money, and is
entirely inadequate for general use.

The other method is to make laboratory tests of the physical properties
of available rocks in a locality, study the conditions obtaining on the
particular road that is to be built, and then select the material that
best suits the conditions. This method has the advantages of giving
speedy results and of being inexpensive, and as far as the results of
laboratory tests have been compared with the results of actual practice
they have been found to agree.

Laboratory tests on road materials were first adopted in France about
thirty years ago, and their usefulness has been thoroughly established.
The tests for rock there are to determine its degree of hardness,
resistance to abrasion, and resistance to compression. In 1893 the
Massachusetts Highway Commission established a laboratory at Harvard
University for testing road materials. The French abrasion test was
adopted, and tests for determining the cementing power and toughness of
rock were added. Since then similar laboratories have been established
at Johns Hopkins University, Columbia University, Wisconsin Geological
Survey, Cornell University, and the University of California.

The Department of Agr