220 R. PUMPELLY— SECULAR ROCK-DISIXTEGRATIOX. 



limestone and sandstone, which incline at a lower angle (15° to 20°) away 

 from the mountain mass. 



Kesting immediately on the altered porphyry is a bed, 10 to 15 feet thick, 

 of angular fragments of ore, with the interstices filled with detritus of decom- 

 posed porphyry, which, showing no signs of stratification, follows the contact 

 in depth. , In places the limestone itself contains rounded pebbles of ore, but 

 more of decomposed porphyry, forming in some beds a conglomerate. The 

 residuary ore of the fragmental mantle-bed is evidently derived chiefly from 

 a oO-foot vein of solid ore, which comes to the surface on the western flank 

 and runs nearly parallel to the strike of the bed of fragmental ore. This 

 bed is part of the pre-Silurian mantle of disintegration, and is not a Silurian 

 sedimentary deposit. This is clear from the facts that it not only shows no 

 stratification, but that the material filling the interstices between the ore frag- 

 ments is wholly decomposed porphyry, without sand or limestone; whereas, 



Figure 4. — Section exposed by Mining on western Flank of Iron Mountain, Missouri, showing Mantle of 

 pre-Silurian residuary Ore under Silurian Limestone. 



a = Decomposed porphyry; 6 = Vein of ore; c = Pre-Silurian mantle of residuary ore, 10 to 15 

 feet thick; d = Silurian limestone ; e = Earth. 



if it had been moved by breaching action, a separation would have taken 

 place, resulting in the removal of the porphyry detritus and its replacement 

 by sand and calcareous matter. 



On the eastern flank. Professor Potter's explorations revealed a pre-Silurian 

 valley, in which a large amount of detrital ore is accumulated, beneath the 

 limestone. The mining has followed this valley for 1,500 feet or more, down 

 its gentle slope, under the Silurian limestones and sandstones. Here also, 

 while the overlying limestone carries more or less debris from the mountain, 

 the ore-bed is unstratified and has its interstices filled with a wash of decom- 

 posed porphyry. The bed is in places 40 feet thick and 300 feet wide, grow- 

 ing narrower toward the lower end of the valley, and thinning out toward 

 the sides, where the limestone rests directly on the porphyry. Toward the 

 lower end of this ancient valley, the ore-blocks are larger and rather more 

 rounded than those further up-stream. I imagine that these lower bowlders 

 are the older ones and started from their source when the parent hill was 



