394 C. W. TOMLINSON 



that every one of them once extended over the whole area and was 

 cut back by erosion before the Mississippian period, is likewise, 

 in all probability, incorrect. 



"Positive" and "Negative" areas; axes of warping. — Whatever 

 was the true proportion between erosion and non-deposition, the 

 net result is shown in Fig. 13, which is, in effect, a paleogeologic map 

 of the central Rocky Mountain region at the close of the pre- 

 Mississippian interval of emergence. If the area of Cambrian 

 and pre-Cambrian rocks which then extended from eastern Utah 

 northeastward across southern Wyoming and northern Colorado 

 owed its exposure to the erosion of Middle Paleozoic formations 

 during emergent intervals, it certainly was more exposed to erosion 

 during those intervals than were the adjacent areas on either side 

 of it — probably because of greater altitude. If its exposure was 

 due wholly to non-deposition of the Middle Paleozoic formations, 

 then it must have been above water when neighboring areas were 

 submerged. In either case, or under any hypothesis which com- 

 bines the two views, this area of Cambrian and pre-Cambrian rocks 

 must have been at a greater average altitude during Middle Paleozoic 

 time than the areas on either side of it. 



The major axis of this area of upward tendency is approximately 

 at right angles to the average trend of the Rocky Mountain folds. The 

 downward- tending belt, which is known to have been covered by 

 most of the Middle Paleozoic seas, extends in parallel fashion from 

 the northern part of the Great Basin northeastward across the 

 Rocky Mountain province. This is well illustrated by the distri- 

 bution of the Bighorn dolomite (see Fig. 9). 



The trend of these belts indicates the direction of axes of warping 

 merely, not of true orogeny. 



The pre-Mississippian interval of emergence was brought to a 

 close by an invasion of the sea, whose deposits are more widespread 

 in the central Rocky Mountain region than are those of any preced- 

 ing submergence. 



