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magnitude, the greater ones corresponding approximately to the folds 

 in the underlying Randville dolomite. The total thickness of the 

 formation is probably 600 to 700 feet. 



The iron ore deposits of large size rest upon relatively impervious 

 formations, which are in such positions as to constitute pitching 

 troughs. A pitching trough may be made [a] by the dolomite forma- 

 tion underlying the I'raders member of the Vulcan formations, [b) by 

 a slate constituting the lower part of the Traders member, and {c) by 

 the Brier slate between the Traders and Curry members of the Vulcan 

 formation. The dolomite formation is especially likely to furnish an 

 impervious basement where its upper horizon has been transformed 

 into a talc-schist, as a consequence of folding and shearing between 

 the formations. 



Unconforraably above the Vulcan iron formation is the Hanbury 

 formation, which forms three large belts in the syncline of the older 

 rocks, and occupies a very large proportion of the district. The for- 

 mation comprises clay slates, calcareous slates, graphite slates, gray- 

 wackes, quartzite, ferruginous dolomite, and rare bodies of ferruginous 

 chert and iron oxide. The formation is much thicker than any of the 

 other formations of the district, but it is probably not thicker than 

 2000 or 3000 feet. 



Wilder' describes and maps the Sioux quartzites and quartz por- 

 phyries of Lyon county, Iowa. No points concerning the stratigraphy 

 or age have been added to those already given by other writers. 



Bain '^ describes the geology of the Wichita Mountains. Gabbros 

 and porphyries of pre-Cambrian and probably of Archean age are 

 present. The gabbro is more prominent in the western portion of the 

 mountains, being especially well developed in the Raggedy Mountains, 

 and the porphyry is more common in the eastern part of the moun- 

 tains, being typically developed at Carrollton Mountain. 



Matthews ^ gives a detailed petrographical description of the gran- 

 ites of the Pike's Peak quadrangle of Colorado. They are referred to 

 the late Algonkian period. 



^Geology of Lyon county, by Frank A. Wilder : Iowa Geol. Surv., Vol. X, 

 1899, pp. 96-]o8. 



- Geology of Wichita Mountains, by H. Foster Bain : Bull. Geol. Soc. Am., Vol. 

 XI, 1900, pp. 127-144, Pis. XV-XVII. 



3 The Granite Rocks of the Pike's Peak Quadrangle, by A. B. Matthews : Jour. 

 Geol., Vol. VIII, 1900, pp. 214-240. 



