FAUNAL EVIDENCE AND CORRELATION. 263 



elusive, ea{>ecially in view of the variation in the mattei- of range of species exhibited 

 in the Leadville and San Juan regions in the case of the earlier formations. 



A general comparison of the geologic column as shown in sections in Colo- 

 rado, Utah, and Arizona has led me to consider the existence of nonsequenco of sedi- 

 ments and life at several horizons in the Carboniferous of these areas. As indicated 

 bj^ variations in geologic sections, one of these occurs at the top of the Leadville 

 and the Ouray limestone, another between the Weber limestone and the Maroon 

 formation, a third between the two divisions of the Maroon, and a fourth between 

 the Maroon and Wyoming formations. The unconformity and erosion found between 

 the Leadville limestone and the overlying Pennsylvanian sediments find faunal 

 expression in Colorado in a complete change of invertebrate life and in the absence 

 of faunas of late Mississippian age. Another striking though not equally strong 

 faunal break is found between the Hermosa and Rico formations, and is further 

 borne out b}^ the existence in Sinbads Valley of a greatly thickened series resembling 

 the Hermosa, which has at its top a fauna more like the Hermosa than the Rico 

 fauna, yet not entirely the same as either. As the Rico formation is supposed to 

 represent the upper Maroon and the Hermosa the lower Maroon, there is some 

 substantial faunal evidence favorable to the third hypothetical interruption. No 

 invertebrate fauna is known from the Trias of Colorado, whose age is determined by 

 vertebrate remains found in the San Juan. As I believe that the Rico fauna is not 

 80 young as the Permian, it is probable that a long interval occurred between the 

 Maroon and Wyoming sediments. The reduction in thickness of the Pennsylvanian 

 portion of the Wasatch limestone from 6,400 feet in Utah to 1,000 feet and less in 

 its supposed equivalent in Colorado, the Weber limestone, suggests that in Utah beds 

 are contained in the Wasatch which are absent in Colorado. Part of this thinning 

 might be ascribed to the ascertained unconformity which preceded the Weber lime- 

 stone of Colorado and which can probably safely be said to be more extensive there 

 than in Utah. On the other hand, it is necessary to consider the possibility of an 

 interruption between the Weber limestone and the Maroon series, the supposed repre^ 

 sentative in Colorado of the Weber quartzite which succeeds the Wasatch limestone 

 of Utah. It is not possible to consider here the faunas of the Wasatch limestone 

 and Weber quartzite. The faunas of the Weber limestone of the Crested Butte and 

 of the Weber shale of the Leadville district have an expression sufficiently 

 distinctive and are sufficiently different from those above to lend some color to such 

 a hypothesis, though doubtless not to prove it. But the faunal evidence tends to 

 correlate the Molas and basal Hermosa formation on one hand with the Weber lime- 

 stone and the Weber shale, and also the persistence over most of the San Juan of the 

 limestones which occur at the base of the Hermosa affords a feature of lithologic 

 correspondence. Man}'' of the distinctive forms of the lower Hermosa are found 



