EFPACEMENT OF ORIGIN-AL GHAEACTERISTICS 251 



the columns. The passage from the compact to the decomposed 

 rock is, in this case, unusually abrupt. Alteration takes place 

 (through the elimination of oxide of iron as before suggested) 

 slowly at the surface, which therefore chips off as soon as de- 

 composed and exposes a new portion. This sudden transition 

 may, in part, proceed from the absence of any natural planes of 

 fracture (which are brought out when there is a concentric 

 structure), and perhaps in part also from the presence of 

 chrysolite. The layer of pseudo-mica schist is in some places 

 five feet thick and has a rusty brownish color. Above it passes 

 into three feet of earth of the same origin, having a brownish 

 black color, and this is covered again by four feet of brownish 

 red soil." 



Such an effacement is not, however, an invariable accom- 

 paniment of decomposition, since where the amount of residuary 

 material is relatively large, and allowed to accumulate in place, 

 the mass may for a long period retain its original structural char- 

 acteristics. Indeed, the original features are sometimes so per- 

 fectly preserved that casual inspection alone quite fails to reveal 

 the havoc that has gone on. Every detail of bedding, jointing, 

 or foliation, or even of internal structure, as brought about by 

 the arrangement or size of the individual particles, may be re- 

 tained with perhaps only a slight change of color due to oxida- 

 tion. This feature is often strikingly conspicuous in the newer 

 railway cuts of the southern Appalachian regions, particularly 

 where the country rock is of the nature of gneisses or schists. 

 In the work of grading the streets, in the extensions of the city 

 of "Washington, masses of strongly foliated granites, so soft as 

 to be readily removed with pick and shovel, would be cut 

 through, which yet showed every vein or other structural detail 

 as plainly marked as in the original rock, and it was only when 

 by jthrusting one's cane or other implement into it that its thor- 

 oughly decomposed condition became apparent. Russell de- 

 scribes^ a similar condition of affairs prevailing in the coarse 

 Triassic conglomerate near Wadesborough, North Carolina. 

 This conglomerate is here composed of rounded and angular 

 pebbles of talcose schist and other crystalline rocks. In the 

 fresh cuts along* the line of the North Carolina railroad, every 

 detail of the original rock is brought out almost as sharply as in 

 the so-called Potomac marble phase of the same formations as 



^Bull. 52, U. S. Geol. Survey, 1889. 



