ORIGIN OF THE UPLIFT 427 



slopes. It has a diameter of approximately 3G miles, with an uplift of 

 about one mile in 10 miles or more. In both areas there were two periods 

 of major folding, with compressive forces of the second period acting at 

 right angles to those of the first period. At the Solitario there was in- 

 tense volcanic action manifested as intrusives and extrusives, while at 

 Marathon tliere was practically no volcanic action. 



Intrusion of a laccolith or other igneous body into the lowest Paleozoic 

 strata at the Solitario may explain the abruptness of the uplift. The 

 sills, dikes, and volcanic breccia would be connected with such a body. 

 Six or more laccoliths in and near the Chisos Mountains southeast of the 

 Solitario have been described by Dr. Udden.^ The Christmas Mountains, 

 near by, are a faulted dome in Cretaceous limestone, one and one-half 

 miles in diameter, uplifted 3,500 feet, and are probal)ly a laccolith, as yet 

 unroofed. Other similar intrusive uplifts are found farther south in the 

 Bufro Mountains of Mexico. 



This hypothesis would explain the circular outline, the steep sides, and 

 ajiparently flat top of the dome. Furthermore, some intrusive body or 

 bodies must underlie the dome, ])ecause of the concentration of dikes, 

 plugs, and sills and because of the vast amount of volcanic breccia which 

 has come from some intrusive which broke through to the surface. It is, 

 however, difficult to conceive of the form and size of an intrusive, even 

 of an intraformational laccolith, in complexly folded rocks which would 

 fulfill the requirements. The laccolithic intrusions above cited seem to be 

 confined to Cretaceous limestones. 



Another hypothesis is that the doming can be explained by tectonic 

 forces without igneous intrusion. An analogous uplift in Tennessee has 

 been called to the attention of the writer by W. A. Nelson, the State 

 Geologist. This dome, the Wells Creek Basin, in Stewart County, near 

 Cumberland City, is two miles in outside diameter and shows an uplift 

 of about one-half mile, with dips around the edges "at high angles and 

 at some points even vertical." ^^ This uplift of Upper Cambrian into 

 Mississippian limestone is 70 miles from the nearest dike in Kentucky. 

 An unconformity is shown in the section at the dome, but not elsewliere, 

 and therefore the dome is a rejuvenation of an older uplift. Tlie doming 

 is clearly the result of tectonic movements free from igneous action. 



"University of Texas, BuHetin 9.3, 1^07; 1753, 1017, p. 148. Also. U. S. Geol. Survey, 

 topographic at!as, Chisos Mountains quadrangle. 



1" .1. M. Safford : fieology of Tennessee, ISfiO, p. 147. par. .S64-r>. 

 .1. .T. (ialloway : Geology of natural resoui'ces of Rutherford County, Tennessee. 

 Geological Survey of Tennessee, Bulletin 22, 1019. 



