192 CARBONIFKROUS FORMATIONS AND FAUNAS OF COLORADO. 



Tlu' •"Ti'ias.sic" or \\y()niiiiL;' f'oriiuitioii seems to possess greater continuity, hut 

 aijpareiiliy it does not extend nuieh soutli of Colorado Sprinj^.s, not being found in 

 the Manilou I'arlc Basin, in tiie Pikes Peak quadi'angle, in tt>e Wet Mountains, or 

 ill the Saiigre de Cristo Range. 



The distribution of tiiese formations woidd indic;ite unconformities hy overhip 

 (and probably by erosion) between the Millsap and Fountain, Ijetween the Fountain 

 and Wyoming, and between the Wyoming and Morrison formations. 



In the San Juan region the sediments representing Upper Carboniferous or 

 Peunsylvauian time are classed as the Molas, Hermosa, and Rico formations. The 

 region from which the Hermosa and Rico formations were first described is the 

 Rico Mountains, but a knowledge of its characters and variations over a much 

 more extended area was drawn upon in framing the first description. The Hermosa 

 formation, which has a thickness of 1,800 feet, is susceptible in the Rico region of a 

 three-fold division. The lowest division consists chiefly of greenish-gray sandstones 

 and shales, the latter being sometimes nearly black. The middle member is charac- 

 terized by many bands of massive, dark-gray limestone, often highlv fossiliferous 

 and alternating with sandstones and conglomerates. The ujjper division is predomi- 

 nantly a complex of shales, with occasional limestones. The constituents of the 

 Hermosa section vary largel}^ from point to point, and the subdivisions which were 

 there practicable can not be distinguished over large areas. The lower division at 

 Rico contains about SOO feet. The beds resting on the Ouray limestone are shales 

 and impure limestones. Above this are gra}' grits or sandstones alternating with 

 gray shales and several beds of black shale and thin, impure limestone. The middle 

 division is made up ver}' largely of blue, bituminous limestone carrying many fossils 

 and occurring in massive beds from 5 to 100 feet in thickness, separated by shales 

 and sandy shales. This division has a thickness somewhat in excess of 600 feet. 

 The upper division contains some bands of limestone similar to those of the middle 

 division, but the}' are thin and vinimportant in comparison. Its strata are mainly 

 black and gray shale alternating with green grits and sandstones. Occasionally 

 reddish sandstones are observed, and two black shales are j)resent in the lower third. 

 The top of the upjaer division and of the Hermosa as a whole is well defined from 

 the base of the next higher formation. The topmost member consists of about 30 

 feet of fine-grained, mica-bearing green shales, immediatel_v above which comes a 

 i"ed, sandy, fossiliferous limestone of the Rico formation. 



The Rico formation is a part of the Red Be'ds series of southwestern Colorado. 

 This series is rather a uniform one lithologically, and somewhat sharply defined 

 from the Hermosa formation below, but the upper portion has been found to contain 

 Triassic fossils, while at the base a well-marked Carboniferous fauna occurs. The 

 upper limit of the Rico formation is arbitrarily taken about oOO feet above its base, 



