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TRANS. ST. LOUIS ACAD. SCIENCE. 



These mountains are composed of porphyries, with various 

 textures, there being no other rock in the immediate neighbor- 

 hood except some low chert and limestone knolls belonging to 

 the Lower Silurian formation ; the limestone is found through- 

 out the valleys, reaching as high up as the 300-foot contour line 

 on the mountains. 



The term " porphyry" was originally applied to any igneous 

 rock of a red or purple color, without any regard to its constitu- 

 ents or structure ; but by the term "porphyry" I mean a compact, 

 microcrystalline, feldspathic base, in which crystals of feldspar 

 or quartz, or both, are developed. 



Nauman proposed to call any feldspathic base, whether it con- 

 tained crystals of quartz alone or associated with crystals of feld- 

 spar, " porphyry," and collect all the quartzless porphyries under 

 the name of " Porphyrite." This nomenclature has been pretty ■ 

 generally accepted, especially in Europe ; and although thinking 

 it the best yet proposed, and being accustomed to it myself, yet, 

 as the members of the Survey, in speaking of porphyry, have 

 understood the rock as first defined, I shall include all under the 

 general name of Porphyry according to their definition. 



In this district there are several varieties of porphyry ; a dense 

 pink-colored porphyry predominates, and is the characteristic 

 porphyry of all that region lying back of Pilot Knob. This por- 

 phyry is usually flesh-colored, but varying in shade, sometimes 

 passing into a deep liver color, with dense matrix, no crystals of 

 quartz or feldspar ; fracture sometimes smooth, again jagged, but 

 generally inclining to half conchoidal : the texture is very varied. 

 A mile east of the Knob, it passes from massive into stratified, 

 slaty, and bedded ; immediately in rear of the Knob, mostly 

 massive, but a few points show magnificent columnar structure. 

 These porphyries seem to be older, and apparently belong geo- 

 logically below the porphyries of Pilot Knob. 



The general character of the Pilot Knob porphyries is con- 

 glomeratic, but the porphyries vary in texture, color, and the ma- 

 terial which acts as a cement. All that mass of porphyry over- 

 lying the ore-bed is a conglomerate, having a granular iron-ore 

 matrix (the iron-ore acting as a cement) containing weathered, 

 angular and rounded fragments of porphyry lying firmly fixed in 

 the mass ; the exposed sui"face presents a bright, metallic lustre, 

 produced by the fine points of iron-ore. On the exposed parts 



