ARCH.^.AN ROCKS OF MISSOURI. 787 



Knob Lick, St. Francois County, and the Ozark quarries near Iron Mountain. 

 At the Knob Lick quarries active work under the present owners began October, 

 1880, and for the year beginning then the product amounted to $100,000. 

 These quarries now belong to Allen & Smith. 



Porphyries are often exposed in Madison, Iron, Wayne, St. Francois and 

 Reynolds, and from the highest peaks in those counties, being elevated from 200 

 to 660 feet above the valleys. The foot of these mountains is generally flanked 

 by porphyritic conglomerate, or else limestone and sandstone of the Potsdam age. 

 The testimony of the rocks goes to show, that, previous to the formation of the 

 sandstone and limestone, the country presented the appearance of rough porphyry 

 knobs rising 1,000 to 1,500 feet above the sea. In these depressions was the 

 Potsdam sea, in its early ages quite tempestuous, as evidenced by the conglomer- 

 ates and coarse sandstone, chiefly formed of eroded fragments from the Archeean. 

 These sandstones occupied the shore line of the Potsdam sea. In the course of 

 time these waters became more quiet and calcareous sediments with occasionally 

 sandy matter were formed; but observation shows that this deposit in no place 

 extends along the Archaean slopes over 350 feet above the present valley. 



These porphyries in their typical and most common form seem to be a fine- 

 grained impalpable mixture of orthoclase and quartz, generally of a red, brownish 

 or purple color, sometimes dark gray or black; are porphyritic chiefly from the 

 presence of feldspar crystals and often of grains of crystallized quartz. Most of 

 the porphyries on their edges show a shade of red. Many of them are banded 

 and show cleavage planes, and in some we find well marked lines of stratification, 

 and some even show ripple marks indicating a sedimentary origin. At Pilot 

 Knob the porphyry incloses rounded pebbles. Epidote, hornblende and ser- 

 pentine occur, also beds and veins of specular iron, represented on a large scale 

 at Pilot Knob, Iron Mountain and Sheppard Mountain; being magnetic at the 

 latter place. 



Slate, resembhng in character roofing slate, occurs on Buck Mountain, Iron 

 County. Dykes of diorite and dolerite sometimes occur. 



At the so called Tin Mountain, in Madison County, the porphyry is traversed 

 by coarse dioritic dykes and black dolerite. On the waters of Captain's Creek a 

 dyke of coarse syenitic greenstone seventy-five feet wide cuts the porphyry. 



In Sec. 16, T. 32, R. 6 E., there is an interesting exhibit of a series of dykes 

 traversing dark porphyry (see figure 8 Mo. Geol. Rep., 1874). Against the por- 

 phyry wall on the east is ten and a half feet of greenstone, next west are a few 

 inches of dolerite, then four feet of porphyry, then two feet of greenstone, then 

 porphyry. The course of dyke is S. 45° W. 



In Iron County in Sec. 9, T. 32, R. 4 E., a dyke of hornblende rock can 

 be traced north and south for one-eighth of a mile; it standing several feet above 

 the surface of the ground, like a wall. 



On Gray's Mountain in Wayne County and in the southeast part of Iron 

 County we find exposed beds of " Steatite." 



