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TRANS. OF THE ACAD. OF SCIENCE. 



Age of the Porphyry Hills of South-east Missouri. 

 By Edwin Harrison, B.S. 



The Magnesian Limestones, interstratified with several 

 beds of sandstone, constitute the sedimentary rocks of Wash- 

 ington and adjoining counties, and, as has been fully de- 

 veloped by the State Geological Survey, belong to the Silu- 

 rian system. Here and there porphyry hills protrude, some 

 of which deserve, as they have received, the title of moun- 

 tains, such as Hughes' Mountain, in Washington county, and 

 Buford Mountain, in St. Francois. 



The relation of these porphyry hills and ranges to the sedi- 

 mentary rocks in contact with them, as regards their relative 

 age, seems never to have been established. Were these hills 

 existing as such before the deposition of the sedimentary 

 rocks which rest against their sides, or were they upheaved, 

 forcing their way through those rocks? As the solution of 

 this question is of some interest as applicable to the geology 

 of a number of counties in this State, I have undertaken it, 

 and, I believe, successfully. 



Two localities in this (Washington) county reveal the por- 

 phyry and magnesian limestone in contact. One of the lo- 

 calities is a point some six hundred yards south-west of Iron- 

 dale, in the east bank of Mill Creek. The section, as seen in 

 the bank, is represented by the accompanying sketch. 



Between the solid mass of porphyry and the beds of lime- 

 stone is a conglomeration of waterworn pebbles and boulders, 

 all porphyry, cemented together by a calcareous matrix show- 

 ing some signs of stratification. Interstratified with the 

 limestone are thin layers containing waterworn porphyry 

 pebbles. The conclusion cannot, therefore, be avoided that 

 the stratified beds took their position in contact with the 

 porphyry at the time of their formation as we now find them, 

 and that, therefore, our porphyry hills existed as such before 

 our Silurian beds were deposited. 



A. — Beds of Magnesian Limestone. 



B. — Pebbles and boulders of porphyry bound together by a calcareous cement with signs 



of stratification, all waterworn. 

 C- — Thin layers in the limestone, consisting chiefly of porphyry pebbles, waterworn. 



