200 T. 8. Hunt—The Decay of Rocks. 
construction, such as the walls of rude chimneys, but at the 
surface it readily disintegrates, yielding a strong red soil, often 
used as a brick-clay. e decayed mica-schists of the Mont- 
alban series, which still praia their micaceous. aspect, hese 
been called hydro-mica Lyrae though distinct from those of 
the Taconian, with w they have been confounde 
24. The 
posits were eae as in each case in d oak of the Montalban 
group—the newer gneisses and mica-schists—and as constitut- 
ing veins or lenticular masses of posterior origin, consisting 
essentially of pyrite, pyrrhotine and chalcopyrite. The agent 
which kaolinized the enclosing rocks, also oxidized the sul- 
phurets, removing the acisnes iat the copper, and converting 
the residue into limonite, which, in a vertical lode in Ashe Co., 
N. C., was found to extend to depths of from forty to seventy 
feet. Beneath the oxidized portion is found in all cases the 
ie anged pyritous mass, seldom carrying more than four 
r five hundredths of copper. The limonites thus generated 
were for some years smelted for iron, both in Virginia and in 
Tennessee, before they were discovered to be gossans at the out- 
crops of cupriferous rites-lodes. etween the unchanged 
bytes and the limonite there is often found in favorable con- 
itions an accumulation known as black ore, consisting of im- 
perfectly crystalline sulphurets, rich in copper, and sometimes 
approaching to bornite in composition, occasionally with red 
oxide and native copper, the whole doubtless reduced from the 
oxidized and dissolved copper brought from above 
§ 25. The crystalline EHozoic roe cks of various ages in 1 the 
more — parts of the continent contain, as is well known, 
of cupriferous pyritous ores, both in veins and 
beds which, ie the enclosing strata, are undecayed, wre 
that the process of oxidation, ihe that of kaolinization, has 
a very gradual one, going back to remote ages. We have ie 
from the observations in the nonuiers: United States that the Ox- 
Proc, Amer. Inst. M. Engineers, ii, 123, and this Journal, vi,.305; see also 
: Chom and Geol. Saavik pp. 217, 250. 
i 
