120 



SOUTHERN APPALACHIAN REGION. 



the Archean, or oldest known age. The different rocks 

 include several kinds of granite, diorite, mica-gneiss, 

 hornblende-gneiss, and various schists. A large number 

 of these ai-e of igneous origin, but the original nature of 

 many of the gneisses and schists is unknown. Formations 

 also included in this group are the ancient volcanic rocks. 

 These are developed most prominently in connection with 

 the Cambrian quartzites in the northeastern part of the 

 mountain district. 



RELATION OF ROCKS TO STTRFACE. 



rocks."*' °" Much of the surface of the Appalachian Mountains is 

 exceedingly ancient. During the later geologic periods 

 it has been subjected to the various natural agencies of 

 destruction and has been worn down according as the 

 rocks presented at the surface were susceptible to these 

 influences. The materials composing these formations 

 are attacked in varjnng degrees by solution and hj chem- 

 ical processes connected with atmospheric and under- 

 ground water. Certain minerals — for instance, carbonate 

 of lime — are readily dissolved hy natural waters, and the 

 rock in wasting away leaves behind only the less soluble 

 portions in forms of clay. To this capability is directly 

 due the reduction of the Great Valley below the mountain 

 mass. Other minerals — for instance, feldspar — are in part 

 dissolved and in pai't chemically altered and decomposed 

 by natural waters, so that the coherence of the rock which 

 contains them is largely destroyed. Two groups in this 

 region have a large proportion of feldspar in their makeup, 

 and their surfaces have been gradually lowered by its 

 breaking down. These are the Ocoee group and the 

 Archean group. A third mineral — quartz — is compara- 

 tively little changed by solution or chemical action near 

 the surface. Formations made up in large part of this 

 mineral retain their altitudes most persistently and are 

 usually the last to be reduced. This composition is most 

 pronounced in the Lower Cambrian group, but is shared 

 also by the Ocoee group and the Archean group. Although 

 the thickness of the Lower Cambrian quartzites is so much 

 less than that of the other groups, their resistance to solu- 

 tion has caused them to remain upheld in ver}" high ridges 

 and peaks. To this are due the clilfs of Chilhowee, Camp 

 Creek, and Iron mountains and the rugged crags of Grand- 

 father. In the case of the Ocoee and Archean groups 

 their immense thickness and the amount of quartz which 



