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 some such materials are necessary. Urban traffic is such as exists on city streets which are not subjected to continuous heavy teaming, but which have to withstand very heavy wear, and need the hardest and toughest macadam rock. Suburban traffic is such as is common in the suburbs of a city and the main streets of country towns. Highway traffic is a traffic equal to that of the main country roads. Country road traffic is a traffic equal to that of the less frequented country roads.

The city traffic will not be considered here. For an urban traffic, the hardest and toughest rock, or in other words, a rock of the highest wearing quality that can be found, is best. For a suburban traffic the best rock would be one of high toughness but of less hardness than one for urban traffic. For highway traffic a rock of medium hardness and toughness is best. For country road traffic it is best to use a comparatively soft rock of medium toughness. In all cases high cementing value should be sought, and especially if the locality is very wet or windy.

Rocks belonging to the same species and