BOOK XVII. III. 27-30
or a light soil be detected by a standard of weight, for what can be understood to be the standard weight of earth? Nor is alluvial soil deposited by rivers always to be recommended, seeing that some plants do not flourish in a damp situation; nor does that much praised alluvial soil prove in experience to be beneficial for a long period, except for a willow. One of the signs of a good soil is the thickness of the stalk in corn, which incidentally in the famous Leborine plain in Campania is so large that they use it as a substitute for wood; but this class of soil is everywhere hard to work, and owing to this difficulty of cultivation puts almost a heavier burden on the farmer because of its merits than it could possibly inflict by reason of defects. Also the soil designated glowing-coal earth appears to be improved by marl; and in fact tufa of a pliable consistency is actually held by the authorities to be a desideratum. For vines Virgil actually does not disapprove of a soil in which ferns grow; Georg. II. 189.and many plants are improved by being entrusted to salt land, as they are better protected against damage from creatures breeding in the ground. Hillsides are not denuded of their soil by cultivation if the digging is done skilfully, and not all level ground gets less than the necessary amount of sun and air; and some varieties of vine, as we have said, draw nourishment XIV. 23.from frosts and clouds. All matters contain some deeply hidden mysteries, which each person must use his own intelligence to penetrate. What of the fact that changes often occur even in things that have been investigated and ascertained long ago? In the district of Larisa in Thessaly the emptying of a lake has lowered the temperature of the district, and olives which used to grow there before have disappeared,
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