DISTRIBUTION OF SEDIMENTARY FORMATIONS. 73 





the numerous faults which run through the region have, however, consid- 

 erably modified this normal distribution. In point of fact, the central por- 

 tion in the latitude of Leadville is mainly covered by the outcrops of Pa- 

 leozoic sedimentary beds and of intruded masses of porphyry, the Archean 

 exposures being confined to deep glacial amphitheaters near the crest of 

 the range, and to minor masses which represent the eroded crests of anti- 

 clinal folds. 



In the northern portion of the area Archean rocks are exposed along 

 the main crest of the range and in the deep canon valleys and glacial am- 

 phitheaters of the streams which flow into the Platte, Paleozoic beds being 

 found only on the eastward sloping flanks of the included spurs. On the 

 western side of the range, owing to the displacement of the great Mosquito 

 fault, the area adjoining the valley of the east fork of the Arkansas is cov- 

 ered by beds of the Weber Grits formation, while a bordering fringe of 

 outcrops of Lower Quartzite and White and Blue Limestone beds is found 

 on the northern and eastern rim of Tennessee Park. 



In the southern half of the map the western limit of Paleozoic beds is a 

 line running southeasterly from the forks of the Arkansas to the crest of the 

 range at Weston's pass, and southward beyond the limits of the map along 

 the crest, approximately in a north and south line. West of this line are 

 found only the granites and schists of the Archean, and irregular dikes and 

 intrusive masses of porphyry. In the area included between this line and 

 the crest of the range are triangular zones of easterly dipping sedimentary 

 beds, in some cases forming a continuous series from the Cambrian to the 

 Upper Coal Measures, cut off abruptly by fault-lines and succeeded again 

 on the east by Archean exposures. On the east of the crest the Paleozoic 

 beds slope regularly back beneath the floor of the South Park, the Archean 

 rocks being found only in the deeper hollows at the heads of the streams 

 Beyond the limits of the map the outcrops of the more resisting beds of 

 Mesozoic age form parallel ridges, running across South Park from north to 

 south. The Quaternary Lake beds are found only along the lower ends of 

 the spurs extending out into the Arkansas Valley from Leadville south to 

 the limits of the map. 



