Under this head are included those accumulations of the mechanical and chemical waste of preexistent rocks which are formed on land surfaces and not in permanent bodies of water. Deposits made by ice are considered in a separate section. The principal agencies which form deposits of this class are rain and wind and springs, and the great variety of them is due to climatic factors, the velocity and constancy of the winds, the quantity and seasonal distribution of the rainfall, the amount and rapidity of temperature changes. Hence we find different kinds of deposits in deserts and humid regions, in cold, temperate, and tropical climates, near the seashore, and in the interior of the continents. The necessity of considering and emphasizing these differences lies in their value for historical studies. Every rock bears within it a record of its history, could we only decipher it.