24 2 



in streaks. Above these lies a thin layer of clay slate, followed by 

 a great mass of bedded iron ore (about forty feet) divided into 

 two parts by a layer of a few feet of clay slate, talcose in parts. 

 The upper portion, which is thin-bedded and flag-like, is less pure 

 than the lower, containing a considerable admixture of silicious 

 matter, and is overlaid by about 100 feet of well-bedded conglom- 

 erate rock, consisting of pebbles or more or less angular fragments 

 of porphyry and gray quartz, in a matrix of granular iron ore, occa- 

 sionally with grains of quartz and a soft clayey matter. In the 

 lower part of this the conglomerate character is less obvious, and 

 it appears to be a uniform ore-bearing porphyry with thin layers of 

 fine conglomerate. The iron oxyd is essentially hematite or per- 

 oxyd, but the rock possesses a decided magnetic polarity. While 

 the great deposit of ore is here newer than the porphyry, and seems 

 to be the cement of a conglomerate made up of the ruins of this 

 rock, it is found in the Iron Mountain in this region', in veins in- 

 tersecting a clayey material, which is nothing but the porphyry de- 

 composed in situ. In a deeper cutting, however, the hard unaltorei I 

 porphyry has been met with. Prof. Pumpelly calls attention to 

 several curious phenomena dependent upon the decay of the crys- 

 talline rocks in this region. In some cases partial decomposition 

 of the granites has left at their outcrop great polygonal rounded 

 blocks, often hundreds of tons in weight. Elsewhere, the chloritic 

 granites for fifty feet, and probably for many times thai depth, are 

 completely disintegrated and decomposed. In the case of the de- 

 cayed porphyry of the Iron Mountain, the effect of the atmospheric 

 waters upon this mass, "part iron and part clay," has been to re- 

 move the latter, so that when the mountain wa fii t oxi lined t 

 exhibited a layer of from four to twenty feet or more in thickness, 

 of rounded masses and grains of pure compact red hematite or 

 specular. ore, with very little clay. This residual detritus, as re- 

 marked by Pumpelly, represents a great amount of porphyry de- 

 composed and removed since the ore-veins bear but a small pro- 

 portion to the whole mass of the rock. In the sediments around 

 the base of the mountain are large stratified accumulations of sim- 

 ilar detrital ore, which were washed down the slope and "concen- 

 trated by the waves of the Silurian ocean," thus showing the great 



antiquity of this process of decay. 



The ore at Cedar Hill near Pilot Knob is compact, holding 

 grains of limpid quartz, and has, according to Pumpelly, the as- 



