SOIL 217 



II. ^Eolian Rocks 



The rocks formed on dry land form very little of the earth's 

 crust, in this respect being altogether insignificant ; their impor- 

 tance lies in 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. If 

 the sand contains any considerable quantity of calcareous matter, 

 the dissolving and redeposition of this by percolating waters will 

 bind the loose material into quite firm rock. 



Talus Blocks gather at the foot of cliffs in large masses ; these 

 may be cemented into a breccia by calcareous deposits, or, by 

 subsidence, may be buried in marine deposits. 



Soil. — In Chapter IV 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 by 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 stand- 

 ing upon them. 



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. 



