THE IRON SERIES, CONTINUED. 



117 



are seamed with one large, and on the 

 west somewhat dome-shaped, parent 

 mass of ore, and innumerable minor 

 veins that radiate into the surrounding 

 rock. Upon the flanks of the porphyry 

 hill rests a mantling succession of sedi- 

 mentary rocks, that dip away on a]l 

 sides. The lowest member is a con- 

 glomerate of ore fragments, weathered 

 porphyry, and residual clay left by its 

 alteration. It is regarded by Pumpelly 

 as formed by pre-Silurian surface dis- 

 integration and not by shore action, 

 inasmuch as sand does not fill the in- 

 terstices, while white porphyry clay 

 does. It is, however, overlain by a thin 

 bed of coarse, friable sandstone, which 

 marks the advance of the sea, and whose 

 formation preceded the limestones. 

 This conglomerate is now the principal 

 source of the ore. It is mined under- 

 ground, hoisted and washed by hy- 

 draulic methods, like those employed 

 in the auriferous gravels of California, 

 and then jigged. The apatite has largely 

 weathered out of it. The rock of the 

 mountain itself, in the cuts of the mines, 

 is largely kaolinized, and exhibits every- 

 where the effects of extreme alteration. 

 The smaller veins that penetrate the 

 porphyry show at times casts or much 

 altered cores of apatite crystals. 



2.02.35. The porphyries of Pilot 

 Knob and Iron Mountain, in thin sec- 

 tion, are seen to belong to quartz-por- 

 phyries, feldspar-porphyries, and por- 

 phy rites. Both orthoclase and plagio- 

 clase are present in them, and many 

 interesting forms of structure. One 

 significant fact is that they are every- 

 where filled with dusty particles of iron 

 oxide, probably magnetite. Our knowl- 



