426 SIDNEY POWERS THE SOLITAKIO UPLIFT 



zoic strata gives approximately the dips of the older series in pre-C*re- 

 taceous time, except for the foreshortening taken up in metamorphism. 

 Therefore the dips must have been 5 degrees to 50 degrees. At Marathon 

 the Cretaceous now dips at angles of 3 degrees to 10 degrees over trun- 

 cated folds which are in part isoclinal. This relation indicates that the 

 major folding in the Big Bend region was pre-Cretaceous. 



The Cretaceous rocks are more gently and regularly folded on the sides 

 of the uplift, the steepest dips being on the north, northeast, and west. 

 The eroded edges of these limestones at the basal contact with the older 

 rocks around the periphery of the dome dip steeply, but on the western 

 side the contact is still preserved, extending within the dome and rising 

 to the top of a ridge of novaculite. Here the degree of dip in the lime- 

 stone diminishes rapidly and becomes gentle and almost flat above the 

 novaculite. This relation suggests that a relatively flat arch of limestone 

 existed over the uplift before the dome was disrupted by igneous intru- 

 sion accomi)anied by faulting. It further suggests that the post-Cre- 

 taceous doming can be compared with the punching upward of a circular 

 core by igneous intrusion. 



Inliers of Cretaceous limestone cap several hills in the volcanic area at 

 the south end of the uplift. Still other fragments of limestone are found 

 in the volcanic breccia, into which they have evidently fallen with the 

 erupted material. The volcanic disturbances have depressed the inliers 

 below the base of the limestone in the Solitario rim. 



The principal inlier is in the center of the dome. The central hill- 

 consist of Cretaceous limestone with a sill at the base resting on novaculite 

 and cut off or overlain by volcanic breccias on the east and south. A dike 

 separates the limestone from the novaculite on the west. The elevation 

 of the base of the limestone on the north is approximately the same as 

 that of the basal contact around the dome, but the dip of the inlier is to 

 the south. Such relationship may be explained in at least two ways : 

 The folding in the limestone roof may originally have conformed to rela- 

 tive competence of formations accentuated by folding and the inlier may 

 be part of an original syncline. On the other hand, this inlier more 

 probably owes its present position to faulting. 



Origin 



The problem is to explain the origin of a circular, flat-topped dome 5 

 mile? in inside and 8 miles in outside diameter, uplifted one mile in a 

 horizontal distance of 2 miles. The Marathon dome is broad, with gentle 



