556 REPORT OF NATIONAL MUSEUM, 1904. 



The porphyry hills, like Pilot Knob and others, he regarded as 

 Azoic rocks, the exposed portions of the skeleton of the eastern part 

 of the Ozark range, rising from 300 to 1,800 feet above the level of 

 the surrounding country. "They form an archipelago of islands in 

 the Lower Silurian strata, which surround them as a whole and sepa- 

 rate them from each other. " The rocks overlying these he regarded 

 as belonging to the oldest members of the Silurian, and thought that 

 they might be the deep-seated equivalents of the Potsdam sandstone, 

 or even older. 



The region of these porphyries, as well as the Ozark range in gen- 

 eral throughout Missouri, had, in his opinion, apparently been above 

 the level of the sea from a very early period (perhaps since the Car- 

 boniferous and Silurian) to the present time.' 



He recognized the fact that the surface ore at Iron Mountain was a 

 residuary product resulting from the disintegration and gradual 

 removal of the siliceous rocks in which the iron ore was originally 

 embedded, the instance offering, as he stated, an extreme case where 

 decomposition of the porphyry in mass facilitated the separation of 

 the ore from the rock and the mechanical removal of the latter. 



Concerning the ultimate origin of the iron, Pumpelly was silent. 

 Regarding the origin of the manganese ore as at Cuthbertsons Hill, 

 in township 33, he wrote: 



It would seem that we have in these occurrences instances of replacement, but it 

 is difficult to imagine a direct substitution of manganese oxides for the decomposition 

 products of a porphyry, and all the more so in this case from the fact that the analysis 

 shows the remaining porphyry, which is intimately associated with the ore, to have 

 its normal constitution. 



He further described what he called a metaphoric limestone at Huffs, 

 near Ackhursts, as — 



nearly wholly changed into a porphyry or jasper rock, it having here a schistoid 

 structure in which the alternate laminae are impure, compact carbonate of lime. 

 * * * Here is a member of the porphyry series which was originally, unques- 

 tionably, a limestone, but in which the original physical and chemical character- 

 istics have almost wholly disappeared. It should not seem impossible that the 

 manganiferous rocks which have been described may have had a similar origin, and 

 that the manganese and iron oxides owe their present existence to a former replace- 

 ment of the lime-carbonate by iron and manganese salts. The porphyry which now 

 surrounds these ores may be due to a previous, contemporaneous, or subsequent 

 replacement of the lime-carbonate by silica and silicates. 



A ; 'may be" to which few would assent at this date. 



A general discussion of the iron ores and their distribution was com- 

 prised in the report of Mr. Chauvenet, which occupied 192 pages of 

 the entire volume. According to this writer the ores of Iron Moun- 

 tain are due to deposition from solution in water which filtered into 

 fissures. Adolph Schmidt regarded these same ores as replacement 

 products and attempted to explain their formation as follows: When a 

 solution of sulphate or chloride of iron containing also carbonic acid 



