720 



SCIENCE. 



[N. S. Vol. II. No. 48. 



way through the Gunnison River into the 

 Colorado, and the greater part is carried to 

 the latter stream through the southward- 

 flowing Slate River and its tributaries. 



The towns of Crested Butte (9,000 feet) 

 and Baldwin (8,750 feet), which are near 

 active coal mines, are reached by branches 

 of the Denver and Rio Grande and the 

 Denver and South Park railroads respec- 

 tively. Other towns higher in the moun- 

 tains, which were founded by silver miners, 

 are Gothic, Pittsburg and Irwin. Owing 

 to its great altitude and abundant precipi- 

 tation, this region is more or less snow- 

 bound during eight months of the year, and 

 mining is thereby rendered difficult and 

 costly. 



Geologic Structure. — The most striking 

 feature in the geology of the region is the 

 great development of eruptive rocks which 

 occur : as irregular bodies cutting across 

 disturbed and uj^turned strata ; as laccolitic 

 bodies doming up the nearly horizontal 

 strata above a given horizon ; as vertical 

 and comparatively narrow dikes ; to a lim- 

 ited extent as surface flows ; and as a bed- 

 ded series of breccias, tuff's and conglomer- 

 ates. 



Eruptive activity was most energetic and 

 widespread during the Eocene Tertiary ; 

 it continued, however, sporadically, during 

 later periods, the most recent outpourings 

 of lava being probably of Pleistocene age. 

 The principal rock types represented are : 

 in the irregular cross-cutting masses, gran- 

 ite and diorite, and at a later period and 

 in limited areas, rhyolite ; the laccolites are 

 mostly of porphyrite ; among dike rocks 

 are found diorite, porphyritic diorite, por- 

 phyrite and quartz porphyry ; basalt occurs 

 as a surface flow, and andesitic debris in 

 the tuffs and conglomei-ates of the bedded 

 series. 



Among sedimentary rocks in this region 

 are found representatives of the principal 

 formations from the Archean up to the close 



of the Mesozoic, with some later formations 

 whose exact age is still somewhat doubtful. 

 The Cambrian is represented by the Sawatch 

 quartzite, which consists of 50 to 200 feet 

 of white quartzite, conglomeritic at the 

 base, and at certain horizons persistently 

 glauconitic ; its fossils are of the Potsdam 

 type. 



The Silurian beds, which are locally called 

 the Yule limestone, in an aggregate thick- 

 ness of 350 to 450 feet, consist mainly of 

 limestones, with quartzite at the base and 

 more shaly beds at the top. They contain 

 the same fish remains that characterize the 

 Harding sandstone of the Canyon City sec- 

 tion, but organic remains have not been 

 discovered in sufficient abundance to admit 

 of the subdivision of the series on a paleon- 

 tologic basis. 



The Carboniferous is represented by three 

 subdivisions. (1) The Leadville limestone, 

 or Lower Carboniferous, has a thickness of 

 400-525 feet of dark gray or blue lime- 

 stones, with some intercalated quartzites 

 and shales. Above this is (2) the Weber 

 formation, which consists of 100-500 feet 

 of shales and limestones, carrj'ing fossils of 

 Coal Measure type. The upper member, 

 known as (3) "the Maroon conglomerate, 

 consists mainly, as its name indicates, of 

 conglomerates, which are characterized by 

 the local abundance of pebbles of limestone. 

 It has an observed maximum thickness of 

 4,500 feet, and in its upper portion resembles 

 lithologically the Red Beds, generally as- 

 signed to the Trias. 



The Jnratrias, whose beds are separated 

 from the last mentioned by a great uncon- 

 formit}', is represented by the Gunnison 

 formation, which consists of a heavy white 

 sandstone, about 100 feet in thickness, over- 

 lain by shales and a little limestone, and 

 carries a fresh water-fauna of supposed 

 Jurassic age. 



The Cretaceous is represented by five 

 recognized subdivisions : The Dakota 



