RECAPITULATION OF PALEOZOIC EOKMATIONS. 215 



that of open water, with similar calcareous sediments. For my own part, these and 

 similar phenomena, such as the fragmentary occurrence of the Sawatch quartzite and the 

 Yule limestone described by the Van Diests, near San Luis, are more naturally 

 explained as remnants of strata that once were continuous over the Archean. In the 

 matter of faunal characters the Leadville limestone at Leadville and the Millsap 

 limestone of the Front Range possess characters in common which distinguish them 

 in some degree from the other Mississippian faunas of Colorado; but since the}^ occur 

 on opposite sides of the long Archean tract of the Colorado range, it seems to me verj"^ 

 doubtful whether this fact can be taken as indicating local conditions due to adjacence 

 to land masses. On the other hand, the dijBference of the Ordovician section along 

 the Front Range and in the interior may afford ground for argument in favor of the 

 existence of a barrier between them during Ordovician time. Yet it would little 

 surprise me, if a careful studj^ of the faunas of both areas were made, to find that 

 the difference is not so great as now ajjpears, and that it is due rather more to lack 

 of uniformity in what has been removed than to lack of uniformity in what was 

 original!}' deposited. 



Attention has several times been called in geologic works to the thinning visible 

 throughout the Paleozoic series in passing eastward from centi'al Utah to the Front 

 Range. This is noticeable even in following the different Paleozoic formations in 

 the State of Colorado. I would attribute this not to a normal thinning or com- 

 pression of the larger series, but to the removal or nondeposition of sediments at 

 many horizons. An instance in point seems to be the absence in Colorado of the 

 upper part of the Mississippian portion and part of the Pennsylvanian portion of 

 the Wasatch limestone of Utah." 



It is a rather significant fact that so many of the formations thin toward the 

 east, and seems to indicate that about the present position of the Front Range, or 

 possibly east of it, exists a permanent line of weakness which has been repeatedly 

 elevated, eroded, and submerged. This has a certain bearing upon a question which 

 has often recurred to me as needing explanation. If, as many believe, mountain or 

 land masses and marine basins have in their general outlines alwaj^s existed, and if, 

 as seems little doubtful, the clastic rocks which form masses of the continental 

 bodies have as their ultimate source the original Archean basement, together with 

 such additional supply as is afforded b}^ volcanic extrusions, or obtained from the 

 atmosphere in the formation of carbonates, it must follow that the original Archean 

 continents were of as large size as the existing ones, unless the land masses were 

 then much higher or the marine basins shallower. The latter proposition seems, 

 indeed, a probable one. If, however, the Archean masses represent permanent 



a The reality of this instance depends of course upon the correctness of my correlation of the Colorado rocks with 

 those of Utah. 



