FERROMAGNESIAN SANDS. 877 



FEin;< > )Lit ; x]-:siax-saxi> family. 



The ferromagnesian-sand family includes the ferromagnesian sands, 

 grits, graywacke, and slate-graywacke, schist-graywacke, or gneiss- 

 graywacke. 



FKRROMACiXESIAN SANDS. 



At many localities ferromagnesian sands are built up which are 

 mainly composed of quartz, the feldspars, and the fei'romagnesian minerals. 

 With these dominant minerals there are subordinate quantities of various 

 other minerals — in fact, any of the minerals which are important as rock 

 constituents, with the possible exception of such readily decomposable 

 soda-bearing minerals as sodalite, nephelite, etc. 



The conditions for the deposition of the ferromagnesian sands are those 

 of disintegration, with little decomposition. These conditions are fully 

 discussed on pages 496—501. It is here only necessary to say that 

 the conditions are substantially the same as those for the formation of the 

 feldspar-quartz sands, except that the rocks from which the minerals are 

 derived contain the ferromagnesian minerals abundantly. In proportion as 

 the climate is arid, in proportion as the material has to be transported only 

 a short distance, in proportion as it is coarse, in proportion as there is great 

 change of temperature by insolation in the warm regions and freezing and 

 thawing in the cold regions, the various igneous rocks are broken down. 

 Thus, sands composed of individual grains or aggregates of grains of the 

 same or unlike mineral composition are formed. The rocks from which 

 the ferromagnesian sands are derived must contain abundantly the ferro- 

 magnesian minerals. These ferromagnesian sands are deposited under 

 physical conditions very similar to those under which the quartz-feldspar 

 sands are deposited. 



The formations may be built up on land or below water. Where built 

 on land in arid regions, thick deposits are likely to result, especially where 

 there is no drainage to the sea. Such deposits are illustrated in the Sahara 

 and in the Great Basin of western United States. The conditions for the 

 formation of sands in such districts have been fully discussed by Walther," 



B Walther, Johannes, Die Denudation in der Wiiste und ihre geologische Bedeutung, S. Hirzel, 

 Leipzig, 1891, pp. 448— 161. Walther, Johannes, Das Gesetz der Wustenbildung in Gegemvart und 

 Vorzeit, Dietrich Rierner, Berlin, 1900, pp. 31-52. 



