848 Oe ISIN 
lithological character, and the resemblance to other beds in the Cor- 
dilleras which have already been so assigned. ‘The Algonkian series 
in areas of strong development has a probable thickness of 3000 to 
4000 feet. It is believed to be unconformable with the underlying 
granite. 
Cross* describes the geology of the Cripple Creek district of Colo- 
rado. The account of the general geology is substantially the same as 
that previously given by Cross for the Pike’s Peak quadrangle,’ of which 
the Cripple Creek district is a part. Granites and gneisses occupy a 
large area in the district. Included in these granites and gneisses are 
large and small fragments of quartzite, quartz-fibrolite-schist, quartz- 
mica-schist, and other similar rocks. It is believed that the quartzite 
fragments belong to a great series of pre-Cainbrian (Algonkian) sedi- 
ments. Hence the granites including such fragments are not Archean ; 
but they are older than the only Cambrian rocks as yet identified in 
Colorado, and they are therefore mapped as Algonkian. The schists 
are probably also sedimentary, but it is quite possible that some, if not 
all, have been produced from Archean gneisses forming the foundation 
upon which the Algonkian sediments were laid down. 
Emmons, Cross, and Eldridge* describe and map the geology of 
the Denver basin in Colorado. Pre-Cambrian rocks form the mass of 
the Colorado or Front Range along the western border of the Denver 
Basin, later formations resting against the flanks of the mountains. In 
the lower canyons of South Boulder and Coal creeks, are beds of 
highly altered quartzite and conglomerate, associated with schists, 
aggregating a thickness of 10co feet, which occupy a position between 
Triassic sandstones and the gneisses of the interior of the range. 
These are undoubtedly sedimentary and are probably of Algonkian 
age. In passing from these sedimentaries westward toward the center 
of the range there appear successively gneisses, granite-gneisses, and 
massive granite. As the areas occupied by the granites and sedimen- 
taries have not been definitely delimited, and as the sedimentaries 
General geology of the Cripple Creek district, Col., by WHITMAN Cross.  Six- 
teenth Ann. Rept. U.S. Geol. Surv., Part II, 1895, pp. 13-109. 
2 Reviewed in this JOURNAL, Vol. IV, 1896, p. 371. : 
3 Geology of the Denver Basin in Colorado, by S. F. EmMons, WHITMAN Cross, 
and G. H. ELpripcr. Mon. U.S. Geol. Surv., No. XX VII, 1896, pp. 556. With 
maps. 
