Hunt.] 116 [Oct 15, 



seen by him at various points in the region to the southwest 

 of Lynchburg, Ya. 



They are principally gneisses with hornblendic and micaceous 

 schists, like those of the Montalban or White Mountain series, and 

 are completely decomposed to a depth of fifty feet or more from the 

 surface, being changed into an unctuous reddish brick-clay, in the 

 midst of which the interbedded layers of quartz are seen retaining 

 their original positions, and showing the highly inclined attitude of 

 the strata. In the adit of a mine, where the rocks had been pene- 

 trated to a considerable distance, the coarsely feldspathic gneiss was 

 found completely kaolinized, but free from the ferruginous coloring 

 of the surface, while farther in, after passing through a partially de- 

 composed portion, the hard unchanged rocks were met with. A 

 similar decomposition of the gneissic and granitic rocks in Brazil, 

 extending to a depth of one hundred feet, has been well described 

 by Hartt, and is known in many other regions. The speaker noticed 

 the permeable nature of the surface-soil thus formed of inclined 

 clayey strata, which affords a natural subterranean drainage, and 

 prevents the accumulation of water in pools and lakes. 



The nature of these chemical changes of the gneissic and horn- 

 blendic rocks was next considered. It consisted essentially in the 

 removal, in the form of soluble carbonates, of the alkalies, lime and 

 magnesia of the silicated minerals, and the hydratation of the resi- 

 dues. The iron-oxyd from these has also been in great part dissolved 

 out by subsequent processes, and was the source of the immense de- 

 posits of hydrous iron ores which are found at the foot of the barrier 

 range of the Blue Ridge throughout the Appalachian valley. 



The great antiquity of this chemical decomposition of the rocks 

 was next alluded to. It was, in his opinion, effected at a time when 

 a highly carbonated atmosphere, and a climate very different from 

 our own, prevailed. That this decomposition had extended to the 

 crystalline rocks to the northeastward, he did not doubt; and he 

 ascribed the absence of decomposed rocks in these regions to a pro- 

 cess of denudation during successive ages, which culminated at the 

 time of the submergence of the northeastern Appalachians at the 

 end of the pliocene period, when the remaining softened material 

 was swept away by the action of water and ice, and the hard, un- 

 changed rocks beneath were exposed and glaciated; since which time 

 the chemical decomposition of the surface has been insignificant. 



