526 A TREATISE ON METAMORPHISM. 



schists of the Piedmont Plateau which, where so soft as to be readily 

 removed with the pick and shovel, still show all their textures and struc- 

 tures. Russell describes the Triassic conglomerate near Wadesboro, N. C, 

 as showing every detail of the original rock, and yet it is so "completely 

 decomposed that when moist it can be cut with a pocketknife through 

 pebbles and matrix alike, as easily as so much potter's clay." a Potter 

 describes the feldspar-porphyry of Iron Mountain as "so soft that it can 

 easily be whittled away with the penknife or scratched with the finger 

 nail; at the same time the original porphyritic structure of individual 

 crystals scattered through the mass is beautifully preserved, and is even 

 frequently more distinctly visible than in the original rock, owing to 

 stronger contrasts of color in the kaolinized material." 6 The so-called 

 tallow rock of the lead and zinc district of southwestern Missouri shows all 

 the textures and structures of the- original chert, yet is easily pared like 

 tallow with the penknife. 



OBLITERATION OF STRUCTURES AND TEXTURES. 



In proportion as the mechanical forces are important in connection 

 with decomposition the original structures and textures are apt to be 

 destroyed because of the differential movement of the particles. Structures 

 and textures are likely to be long preserved in cases of decomposition 

 alone, but are destroyed by decomposition and disintegration combined, 

 since these act together at and yery near the surface. 



The final stage in the process of weathering is the destruction of all 

 textures and structures and the production of soils and subsoils. Near the 

 surface, after the various minerals are broken apart they are mixed with 

 one another indiscriminately by the mechanical forces of freezing and 

 thawing and by the work of plants and animals (see pp. 440-451), and they 

 are decomposed by the chemical forces. Finally no definite structures and 

 textures remain. The character of the surface soil is modified by its 

 organic content. It may contain from a small percentage of organic mate- 

 rial to a predominant amount of that material, as in the case of the peat 



« Russell, I. C, Subaerial decay of rocks, and origin of the red color of certain formations: Bull. 

 U. S. Geol. Survey No. 52, 1889, p. 16. 



6 Potter, W. B., The iron-ore regions of Missouri: Jour. United States Assoc. Charcoal Iron 

 . Workers, vol. 6, 1885, p. 25. 



^Merrill, George P., Rocks, rock-weathering, and soils, Macinillan Co., New York, 1897, 

 pp. 264-265. 



