The rocks formed on dry land form less of the earth's crust than do the aqueous rocks, but they have a special importance because of the hints which they often give as to the physical geography of the place and time of their formation.

Blown Sand is heaped up by the wind into dunes, and displays an irregular kind of stratification. The sand-grains, abraded by their contact with hard substances, are smaller, more rounded, and less angular than the grains of river or even beach sands.

Drift-sand Rock (also called aeolian rock) is the consolidated sand of dunes. If the sand contains any considerable quantity of calcareous matter, the solution and redeposition of this by percolating waters binds the sand into quite a firm rock. The calcareous sands of Bermuda give an often quoted example of this.

Talus gathers at the foot of cliffs in large masses, and in many deserts it forms great sheets.

Breccia is a rock composed of angular fragments cemented by deposition of material, commonly CaC03, in the interstices; the fragments may be any kind of rock. Breccia is also found in zones of fracturing and shattering of the rocks along fault-planes, and is then called a fault-breccia.

Soil

In Chapter IV (Destructive Processes - The Atmosphere) it was shown that soil is mainly the residual product left by the atmospheric decay of rocks, and that its surface layers contain more or less organic matter and are filled with the roots of plants. Soils may be buried under aqueous deposits by floods, or after subsidence marine deposits may be built up upon the soils, which are then interstratified with marine rocks. Ancient soils have been frequently preserved in this manner, filled with fossil roots, and sometimes with the stumps of trees still standing upon them.

Loess

A very fine grained terrestrial deposit, usually unstrati-fied and with a vertical cleavage. It is quite firm and may even become hard and stony (see p. 190).

In logical order, the Metamorphic Rocks would next come up for consideration; but since we have, as yet, learned nothing of the processes by which these rocks are formed, it will be best to defer the study of this class to a future chapter, when the rocks and their mode of formation will be examined together.