184 TRANS. ST. LOUIS ACAD. SCIENCE. 



afterwards became conviction, as to the process by which these 

 ores were deposited. Cedar Mountain is, in the greater portion, 

 a mass of Porphyry, more or less dense in texture, with little or 

 no quartz ; the lesser portion is a conglomeratic porphyry mass. 

 Wherever a section of this conglomeratic mass is exposed to view 

 it shows irregular deposits of iron-ore, which, when carefully ex- 

 amined, seem to be younger than the conglomerate, and the con- 

 viction is forced upon the examiner that the deposit has been 

 formed by replacement, and that some solvent has gradually re- 

 moved the porphyry and at the same time deposited the iron-ore 

 in the vacated place : this could not have been accomplished in 

 the present physical condition of the mountain, as the ore extends 

 to the very highest point, and it is not probable that a solvent 

 would force its way upwards unless acted on by some subterra- 

 nean force ; then, for an explanation, we must imagine the pres- 

 ent localities of the ore-deposits to have been at a much greater 

 depth during the formation than they at present occupy. This 

 being granted, the theory of a replacement of the porphyry by 

 the ore through lateral-secretion would fully account for the ori- 

 gin and present condition of the ore-bed. 



Being convinced of the process by which the ore-deposits were 

 formed at this locality, I turned to Pilot Knob to see if this the- 

 ory would not account for the formations there. But the question 

 was forced upon me to inquire why there was so much regula- 

 rity in the latter formation — if a solvent passed through the 

 conglomeratic mass, why was the porphyry dissolved in one di- 

 rection and always with an average thickness, the whole mass 

 being chemically and geologically the same. But here arises 

 another question, xvere they homogeneous? Above and below 

 the bed of ore lie porphyry conglomerates ; but was the space 

 now occupied by the ore at one time filled by the same overlying 

 and underlying porphyry conglomerate, or by some less durable 

 substance with a slaty or bedded texture ? If the latter, then the 

 problem was easily solved, because a solvent percolating the mass 

 would be most likely to attack that material which offered least 

 resistance. A stratified or bedded porphyry would offer much 

 less resistance to a solvent than a dense homogeneous porphyry. 

 These questions and their answers occurred to me, and I imme- 

 diately sought for some sign of bedded porphyry, but on the Knob 

 found none. I looked further, and discovered east of the Knob a 

 singular formation of a slaty texture, with dip and strike, though 



