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pect of a porphyry, in which the whole matrix has been replaced 

 by iron ore. This forms irregular masses in ordinary porphyry, 

 which in other localities contains iron ores highly manganesian, and 

 even deposits of nearly pure oxyd of manganese. Crystals of or- 

 thoclase, feldspar and grains of quartz, are found imbedded in a 

 compact manganese ore, which, according to Pumpelly, may be 

 supposed to have replaced the matrix, leaving the crystalline ele- 

 ments intact, while in other portions the replacement has been 

 complete, manganese-oxyd taking the place of the grains of 

 quartz, and the feldspar crystals. With these manganiferous por- 



of pink and greenish crystalline limestone several inches in thick- 

 ness, interlaminated with a schistose jaspery or porphyroid rock. 

 To account for these various associations, Prof. Pumpelly suggests 

 two hypotheses, the one that the porphyry, both matrix and in- 

 cluded crystals, may have been replaced by oxyd of iron or of 

 manganese, and the other that the parent rock may have been a 

 limestone, parts of which were changed into ore by a similar re- 

 placement, " while the porphyry now surrounding the ores may be 

 due to a previous, contemporaneous or subsequent replacement of 

 the lime-carbonate by silica and silicates." The important fact is 

 noted that chemical analysis shows that the remaining porphyry, 

 intimately associated with the ore, has undergone no change, but 



a great irregular vein of specular ore, more or less split tip, and 

 including masses of wall-rock, but accompanied by numerous 

 smaller veins. He supposes the ore to have been deposited in fis- 

 sures in the unaltered porphyry, which was further cracked and 

 assured by the crystallization of the ore, while this was itself sub- 

 sequently broken by the contraction and the decomposition of the 

 porphyry ; in fact, the angular fragments of ore in the latter can 

 scarcely be otherwise explained. The writer can, from his own 

 observations, bear witness to the careful statements of facts in 

 the case of these curious ore-deposits as given in the present vol- 

 ume, and affirm that the singular perplexity of the phenomena at 

 the Iron Mountain can seareely be bettor described or explained 

 than has been done by Dr. Schmidt. As regards the origin of 

 the ore-deposits Dr. Schmidt considers the various hypotheses of 

 igneous injection, of sublimation and of segregation, and rejects 



