GAGE — IRON ORES IN MISSOURI. 185 



not precisely the same, yet similar to the ore-beds of Pilot Knob : 

 this formation passed from ferruginous slates almost to pure 

 seams of iron, and in fact the formation lying in the porphyry hill 

 contained in the same bed specular-ore. 



These iron-slates were no doubt at one time first banded, then 

 stratified porphyry ; then the texture became bedded ; finally 

 slaty : and when it reached this stage, and was ready to be easily 

 acted upon, the solvent came, dissolved the porphyry slates away, 

 and replaced in their stead these ferruginous slates and iron-ore. 



I felt fully warranted in drawing this conclusion from the facts 

 already gathered ; but to remove all doubt, and make the chain 

 of evidence complete, another link was wanting. I had seen the 

 gradual change from porphyry slates into ferruginous slates and 

 these into ore-beds, but as yet no indication of the origin of the 

 porphyry slates. The last link needed was furnished in the ra- 

 vine on the south-west slope of Buzzard Mountain ; here there is 

 a gradual transition of texture from banded into stratified, strati- 

 fied into bedded, bedded into slates : thus the chain is complete, 

 and fully accounts for the bedded texture of the Pilot Knob ore. 

 And in order to fully understand all the changes we have but to 

 imagine the present position of the ore-bed, when occupied by 

 slates, to have been situated at a great depth ; and by replacement, 

 according to the theory of lateral-secretion, the slates disappeared, 

 the ore-beds were formed, and later, through some subterranean 

 force, the whole formation, including the ore-beds and the over- 

 lying and underlying conglomeratic porphyries, were slowly ele- 

 vated to their present position. 



Having examined into the origin and present position of the 

 ore-deposits found in the porphyries, we will glance at their min- 

 eralogical and chemical properties. These iron-ores may be 

 divided into three classes— Magnetite, Specular Iron, and Man- 

 ganiferous Iron. The magnetite is the richest iron-ore, yielding 

 72.4 per cent, of the metal. Magnetite is found only in one local- 

 ity to any extent, occurring in veins on Shepherd Mountain, the 

 enclosing rock being a red porphyry with a dense matrix contain- 

 ing crystals of a triclinic feldspar with no free quartz ; the vein- 

 formation is the predominating occurrence of the ore in this 

 district. 



The specular-ore occurs much more extensively, being found 

 throughout the whole porphyritic district, with a variety of tex- 

 ture : in beds, as on Pilot Knob ; in true veins, as on Shepherd 



