BROADHEAD AGE OF OUR PORPHYRIES. 369 



darkest colored will show a reddish tint on a thin edge, if viewed 

 with a magnifying glass. Some of them contain particles of both 

 quartz and feldspar imbedded in a feldspathic paste. Many of the 

 porphyries are banded, and show cleavage planes. Sometimes 

 well marked lines of stratification are seen on a grand scale. 

 Fragments are in my possession that appear to be ripple-marked, 

 but I could not positively so decide unless I were to see the body 

 from whence they were obtained, for this may be due to flexure 

 of strata. Epidote, hornblende, and serpentine occur, and beds 

 and veins of specular iron ore are of frequent occurrence. These 

 veins also occur in the granite of Madison County. Slate (resem- 

 bling roofing slate) is found at one place in Iron County, although 

 in beds too rough to be useful. Quartz veins occur both in the 

 granite and porphyry. A black magnetic iron sand is often found 

 washed out and lying along the surface in the vicinity of the 

 greenstone dykes. 



Our Archaean rocks contain neither gneiss nor limestone : all 

 of our limestones may be referred to more recent age. 



Beds of coarse conglomerate are often seen resting on these 

 rocks. These conglomerates are also chiefly made up of rounded 

 fragments of Archaean rocks. 



Besides the conglomerates just named, it is not uncommon to 

 find shales or slate entirely composed of porphyritic debris, and 

 resting on unaltered porphyry. 



I have examined the porphyries over most of Iron and Madison 

 Counties, also some of those of Wayne, Reynolds and St. Fran- 

 9ois Counties. The granites chiefly occur in the northern portion 

 of this Archaean region. That near Iron Mountain is a coarse red 

 variety, looking well when polished. Near Knob Lick are both 

 the red and the gra}'. South of Burns Mountain, in Madison 

 County, is a very coarse granite. At the latter place, it touches 

 the porphyr\'. Along the northern part of Madison County, both 

 granite and porphyry appear, but at no place have I seen these 

 rocks under such circumstances as to say that one was older than 

 the other. One strong evidence in favor of the older age of the 

 granite, is, that the older unaltered magnesian limestones are often 

 found resting on beds of porphyry, or filling valleys between por- 

 phyry hills ; but I have nowhere seen these limestones reposing 

 on granite. I have seen these unaltered limestones reposing on 



iii — 24 [May 31, 1S76.] 



