$42 CI, ILIEIITE 
areas are the Graniteville and Stout’s Creek or St. Francois areas. The 
porphyries occur in numerous large, uniformly distributed areas, mak- 
ing up nearly half the area of the entire sheet. They include what 
have been called by other writers quartz-porphyry, feldspar-porphyry, 
felsite, felsophyre, and orthophyre. 
Numerous observations show gradations between the granites and 
porphyries, and it is concluded that the granite and porphyries were 
formed from the same or similar magmas, and that their difference in 
texture is due to crystallization under different conditions. 
Algonkian rocks are found near the center of the area, capping the 
Archean rocks of Pilot Knob. They comprise conglomerates and 
slates, chiefly the former, and include the iron ore deposits of the 
locality. The pebbles of the conglomerate are mostly derived from 
the porphyry. ‘The matrix is a fine felsitic mass mixed intimately with 
varying amounts of hematite. In places the ore forms almost the 
entire body of the rocks. 
Paleozoic rocks unconformably overlie the crystallines, and dip 
away from the Archean hills. 
Keyes and Haworth* describe and map the geology of the Mine Le 
Mot sheet, which includes portions of Ste. Genevieve, Madison, and 
St. Francois counties, Missouri. Archean rocks, described by 
Haworth, occupy about half of the area of the sheet, forming the 
nucleus about which later formations are exposed in concentric belts. 
They are granites and porphyries, cut by dikes of diabase. ‘The acid 
rocks greatly predominate, the granite making up fully nine tenths of 
the eruptives of the area. The porphyry appears to be the surface 
facies of the granite, and seems to graduate downward into the latter. 
This is shown where erosion has been great, and has left high granite 
hills which are often capped by porphyry. 
Cambrian rocks directly overlie the Archean rocks, with uncon- 
formable relations. 
Keyes,? considers the granites and porphyries in the eastern 
part of the Ozarks. Agreeing with Haworth, he finds the granites 
*Report on the Mine Le Mot sheet-- General geology, by C. R. KEYEs; 
Archean geology, by C. R. KEvyEs and E. HawortTH. Rept. Missouri Geol. Survey, 
Vol. IX, 1896, pp. 14-44. With Sheet No. 4. 
2 Geographic relations of the granites and porphyries in the eastern part of 
the Ozarks, by C. R. KEyEs. Bull. Geol. Soc. Am., Vol. VII, 1896, pp. 363-376. Pl. 17. 
