WHITE.] SILURIAN SYSTEM. 23 



The columns of this table are arranged in the order of time of their 

 publication, beginning at the left. The horizontal arrangement of the 

 spaces within which the names of the groups of strata are placed is in- 

 tended to represent their correlation; while the spaces themselves rep- 

 resent simply approximate coordination, the relative breadth of each, 

 space having no reference to the relative thickness of each group. 



THE SILURIAN SYSTEM. 



No other strata than those of the Weber Quartzite in this district are 

 referred to Silurian age ; and, as has already been shown, this great 

 formation has been only provisionally so referred. 



THE WEBER QUARTZITE. 



This formation is usually of a dull brick-red or reddish-brown color, 

 and has a peculiar sombre aspect, especially as seen in the deep canons 

 that have been eroded through it in the Uinta Mountain region. The 

 stratification is always distinct, the strata more or less regularly bedded; 

 sometimes massive, sometimes in thin layers, and occasionally in the 

 condition of sandy shales. Sometimes it is in the condition of a com- 

 mon, but firm, sandstone, the grains being distinctly definable ; but 

 oftener the grains are so compacted by partial metamorphism as to give 

 the rock the character of a true quartzite. In its common sandstone 

 condition it is almost always harder than the sandstones of the Carbon- 

 iferous and Mesozoic groups of that region usually are. So far as I 

 am aware, no fossils of any kind have ever been discovered in any part 

 of tihis great formation ; and its true geological age is therefore un- 

 known. 



Upon his map, before referred to, Mr. King assigns this group to the 

 Carboniferous system. 



Dr. Uayden, several years ago, suggested the Silurian age of this 

 great group, as did also Professor Marsh, who visited the region after- 

 ward. With this judgment, I am disposed to agree ; but as before in- 

 timated, its geological age cannot be definitely known without the aid 

 of fossils. Professor Powell has suggested the Devonian age of the 

 Weber Quartzite, in connection with his statement of the unconform- 

 ity of the true Carboniferous strata upon it. With this statement as 

 to its unconformity with the Carboniferous, I also agree; but accord- 

 ing to my own observation the unconformity is usually so slight that 

 it may easily be overlooked. Furthermore, at all observed places in this 

 district, as well as within a large area outside of it, the strata of the 

 Lower Carboniferous rest directly upon the Uinta sandstone, those of 

 the Lodore Group being absent. The Lower Carboniferous strata are 

 often so very like those of the Weber Quartzite in general aspect that 

 a casual observer would be in danger of confounding one with the other, 

 especially if the unconformity should be, as it generally is, slight or 

 obscure. 



There are two or three localities, just where some deep dry-drainage 

 canons open into Red Rock Basin at the north side of Yampa Plateau 

 by cutting across the Yampa Flexure there, where some of the upper 

 strata of the Weber Quartzite are probably exposed ; but for reasons 

 already stated I am not entirely satisfied on this point, and therefore 

 refer those strata to the Lower Carboniferous. 



Strata of this group are well exposed in the walls of the canon by 

 which the Yampa passes through Junction Mountain, and they are also 

 largely exposed in the southern half of Yampa Mountain, which lies 

 some fifteen miles to the eastward of Junction Mountain. 



