252 THE PHYSICAL MANIFESTATIONS OF WEATHEEING 



used in the Capitol building at Washington. ''On examining 

 more closely, however, one is surprised to find that it is com- 

 pletely decomposed, and that when moist it can be cut with a 

 pocket knife through pebbles and matrix alike, as easily as so 

 much potter's eky. The full depth of the alteration in this 

 instance is not revealed, but it extends more than 30 feet below 

 the surface without change in character, ' ' 



W. B. Potter described^ the feldspar porphyry of Iron Moun- 

 tain, Missouri, as decomposed to the extent that it can be easily 

 whittled away with a penknife or scratched with the thumb nail. 

 ''At the same time," he writes, "the original porphyritic 

 structure of the 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.'' In many dense massive rocks, 

 indeed, such features as flow structure and inequalities of text- 

 ure are rendered evident only on weathered surfaces. The same 

 is often true of fossiliferous limestones, a weathered surface re- 

 vealing the presence of organic forms wholly imperceptible on 

 one freshly broken. 



The crude kaolin as removed from the pits near Brandywine 

 Summit, Pennsylvania, and at Hoekessin, Delaware, still retains 

 more are less distinctly the structure of the original gneiss or con- 

 glomerate from whence it was derived. The quartz granules 

 of the gneiss are, in these cases, almost invariably shattered, 

 as though crushed by dynamic agencies, and show distinctly 

 corroded surfaces, presumably caused by the alkaline carbo- 

 nates formed during the kaolinizing of the feldspars. The 

 black mica makes its former presence known by rust-colored 

 spots which, in those cases where the mineral was sufficiently 

 abundant, have ruined the material for the purposes of the 

 potter. 



(11) Simplification of Chemical Compounds, incidental to 

 Weathering. — It has been noted on p. 150 that the process of 

 weathering is but an attempt on the part of the elements in 

 their various combinations to adjust themselves to existing con- 

 ditions. This adjustment consists in the formation of new com- 

 pounds which are characterized by a less complex structure than 

 those first formed. 



Indeed, one of the most striking features of chemical geology 



* Jour. IT. S. Assoc, Charcoal Iron Workers, Vol. YI, p. 25. 



