ARCH ALAN ROCKS OF MISSOURI. 735. 



GEOLOGY- 



ARCH^AN ROCKS OF MISSOURI. 



BY G. C. BROADHEAD. 



This includes the Granites and Porphyries and their associated and intrusive 

 beds in Southeast Missouri. The granites are generally coarse-textured, feld- 

 spathic and quartzose, deficient in mica; are red or else of various shades of gray 

 light or rich gray, or blending into a reddish gray. They crop out in massive 

 beds in the northern portions of Iron and Madison and the southern part of St. 

 Francois County^ with isolated exposures in St. Genevieve and Crawford. They 

 afford our best quality of building stones. In some localities we find evidence of 

 disintegration and decomposition on a grand scale, as for example, eight miles 

 west of Fredericktown. At this place Mr. A. Tucker sank a well seventy five 

 feet deep passing through granitic sand. 



At Lloyd's, in the western part of Madison, south of Blue Mountain, are also 

 found evidences of considerable disintegration. These are probably due to 

 chemical causes and must not be brought forward as evidence against the general 

 use of granite. A majority of the outcrops show massive beds with no disintegra- 

 tion. We must also remember that these rocks have been exposed to the action 

 of the elements through a long course of ages, and viewing them thus, we find 

 that the disintegration of more recently formed rocks is much greater. 



The phenomenon of rocking stones is finely exhibited near the Ozark granite 

 quarries, four miles southwest from Iron Mountain, while other beds near by are 

 firm and appear resistless to the continued action of atmospheric agency. 



In the northern part of Madison County, east of St. Francois River, granite 

 appears over an undulating district, generally of a gray color, porphyritic near the 

 Iron Mountain Railroad. West of the St. Francois River the red granite rises 

 into mountain peaks, as Huckleberry Mountain, Bellmar Mountain and Stone 

 Mountain. The latter on the south is a porphyritic granite. A syenitic granite 

 forms a "shut-in"^ on the St. Francois River near the Einstein Mines, and 

 forms the "rapids" on the river. At this place it is traversed by a dyke of 

 black dolerite forty-four inches wide, bearing S. 60" W. A few miles north, on 

 the banks of St. Francois River the granite contains numerous specks and scales 

 of micaceous iron and also much iron pyrites. We find it here overlaid by hori- 

 zontal beds of sandstone and conglomerate. 



* " f hut-in," a local term signifying that steep rocky cliffs approach close to each bank of the stream. 

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