196 IRVING. 



and the problems that have arisen from their study is here intro- 

 duced. 



At the base of the stratigraphic column is a series of Algon- 

 kian slates, schists, quartzites and amphibolites ; everywhere 

 tilted at a high angle and showing an advanced state of meta- 

 morphism. Lying unconformably upon the eroded surface of 

 the Algonkian is a series of calcareous sandstones, shales and 

 limestone breccias which show a thickness of something over 

 four hundred feet. In the localities where intruded sheets are 

 numerous this thickness is greatly increased. Above these and 

 lying in apparent conformity with them are the heavy -bedded 

 limestones of the Carboniferous, which attain a thickness in 

 the district mapped of upwards of 500 feet. Above this lies 

 125 feet of white and red, variegated, Minnelusa sandstones, of 

 disputed age. They are generally regarded as Upper Carbo- 

 niferous. This would then give to the massive strata of the Car- 

 boniferous age a total thickness of 625 feet. Certain shaly 

 horizons exist in its limestone, but they are of minor importance, 

 and as compared to the underlying Cambrian formation, the Car- 

 boniferous exhibits a very homogenous, heavy-bedded series. 

 The Minnelusa sandstones are not exposed in the district mapped. 



A. DESCRIPTIVE. 

 1. Algonkian. 



The rocks of the Algonkian which are exposed in this area 

 are garnetiferous mica schists ; micaceous slates, which grade 

 into extremely fine phyllites ; argillaceous slates ; finely bedded 

 and greatly indurated quartzites or quartz-schists ; and finally 

 amphibolites and hornblende schists of endless variety. The 

 series is exposed in the vicinity of Central City, Texana and 

 White-tail gulch. It passes beneath the overlying Cambrian on 

 the west and south before we arrive at the towns of Portland 

 and Terry. To the north and east it disappears in the vicinity 

 of Sheeptail gulch and Garden City. The Algonkian exposure 

 extends as a long tongue up into White -tail gulch before it is 

 buried by the palaeozoics and outcrops appear likewise in Fan- 



