RECAPITULATION OF PALEOZOIC FORMATIONS. 213 



limestone with Upper Carboniferous fossils. The fauna of the pebbles, however, 

 is not that of the Weber limestone (and of course not that of the Leadville), but of the 

 Maroon itself. Between the Hermosa (= the lower Maroon) and the Rico (supposed 

 to equal the upper Maroon) another interval may exist. This is indicated hy the f aunal 

 change between the Hermosa and Eico formations, bj^ the much greater thickness of 

 the Hermosa in Sinbads Valley, and the presence in its upper portion of a fauna dif- 

 ferent from that of the typical Hermosa. Evidence favorable to this, however, ma^^ 

 exist in the color change to the dark-red Maroon beds, and in the great number and 

 size of the Ai'chean bowlders which their conglomerates contain. Between the Rico 

 and the upper Maroon formation on one hand, and the brick-red Dolores of the San 

 Juan, the Triassic of Aspen and elsewhere in Colorado, and the Wyoming formation 

 of the Tenmile district on the other, another important interval, probably accom- 

 panied by erosion, seems to have occurred. This is indicated by the probable 

 absence of the Permian and possibly of portions of the Pennsylvanian series between 

 the Rico and Dolores beds, and by the overlap of the "Triassic" upon the Archean 

 at a number of points, and is implied in the suggested correlation of the section in the 

 Uinta Mountains with that of central Colorado. Owing to it the whole Carboniferous 

 series in the Uinta Mountains above the Uinta sandstone is missing elsewhere in Colo- 

 rado," and the upper Maroon from a thickness of 2,500 feet in the northern part of the 

 Crested Butte quadrangle is greatly reduced in the southern, becomes only 300 feet 

 in the San Juan (Rico formation), and is apparently missing altogether in the Dolores 

 River region. I may have misread the paleontologic and other evidence in some 

 cases, and have erred in correlating the Colorado sections with one another and with 

 those of the Uinta and Wasatch mountains (though the latter correlatidn is certainly 

 borne out by numerous collateral circumstances), but the existence of an unconformity' 

 and time break following the Leadville horizon can hardly be questioned. Now, it 

 seems to me that the conditions under which neither erosion was carried forward nor 

 sedimentation going on (except for abyssal tracts which can probably be left out of 

 the discussion) must have been rather exceptional, and that their existence over large 

 areas and permanence through protracted periods of time must have been of I'are 

 occurrence. The apparent conformity in Colorado in local sections of Paleozoic 

 formations showing important differences of geological age would then indicate 

 either protracted erosion and complete baseleveling or slight elevation and shorter 

 periods of exposure to atmospheric forces. Somewhat as it is true that the oldest 

 mountains are the lowest, may it almost be said that the greatest unconformities, 

 those involving the most extensive territory and the longest periods, are least appar- 

 ent. Where the ai'eas which have been tilted are extensive, the angular unconform- 

 ity between the existing and the succeeding sediments is often imperceptible. Where 



« Unless they are represented in the northern part of the State I)y sediments now supposed to belong to the Maroon 

 Weber series. 



