GAGE — IRON ORES IN MISSOURI. 1S9 



of the outcrop the porphyry is very much weathered and shows 

 little structure ; but on a fresh surface, where the fracture is very 

 ragged and uneven, a greyish-white weathered porphyry is seen 

 lying, breccia-like, in the mass. 



On the south-west side of the Knob is a large outcrop of a light 

 purple colored, lustreless, earthy porphyry, very much weath- 

 ered ; and farther down, an accumulation of a white steatite min- 

 eral in a conglomeratic porphyry : this mineral is probably a 

 product from the decomposition of the feldspar in the porphyry. 



Immediately underlying the conglomeratic porphyry which 

 forms the top of the Knob is a bed of lean iron-ore, or flag-ore. 

 16 feet in thickness ; this flag-ore is composed of alternate layers 

 of silicious iron-ore and a light liver-colored ferruginous porphy- 

 ry, the layers of neither being less than -L and not more than } of 

 an inch in thickness. The flag has a shit)- texture, and cleaves 

 with an even granular fracture parallel to the bed ; with more 

 difficulty it breaks in the direction of the strike, and when broken 

 presents a fresh, granular, uneven fracture. The flag also pos- 

 sesses a jointed structure (similar to the columnar porphyry 

 east of the Knob and the plate porphyry on Buzzard Mountain), 

 being cleft internally by Assures or joints ; the jointing is very 

 regular, the clefts or fissures observing one general direction, 

 causing pieces severed from the mass to form regular plates and 

 columns, the axes of the jointed plates being at right angles to 

 the plane of the larger cooling surface, or at right angles to the 

 plane of bedding. When first seen, I thought this flag must be 

 of sedimentary origin, having originally been deposited in hori- 

 zontal layers, then depressed to a great depth, subjected to heat 

 and afterwards elevated, and by unequal cooling had imparted 

 to it this jointed structure ; but I now think that the porphyry 

 was formed by igneous action. Being at great depth, and under 

 great pressure, different textures were assumed as the force most 

 favorable to the formation of the massive or bedded structure pre- 

 dominated. This bed, now a flag-ore, which was at that time a 

 porphyry, on cooling assumed a plate and columnar texture ; and 

 when the porphyry was removed, and replaced through Lateral- 

 secretion action by the present occupants, the structure or texture 

 was undisturbed. 



Immediately underlying the flag-ore is the iron-ore bed proper, 

 separated from the former by a clay-slate averaging two feet in 



