INTRODUCTION 419 



The most remarkably symmetrical dome attracted the attention of the 

 writer in 1919, in passing, and caused him to spend several days within 

 it in 1920 in part in the company of Wallace E. Pratt, to whom he is 

 indebted for many suggestions. Mr. W. S. Adkins and others of the 

 Bureau of Economic Geology, Austin, Texas, have studied the area in 

 detail, under the direction of Dr. Udden, and the present brief descrip- 

 tion is preliminary to their report. 



Summary 



Geologically, the Solitario consists of an unroofed, steep-sided, flat- 

 topped dome formed of Lower Cretaceous limestones, within which there 

 is an eccentric dome of steeply tilted Ordovician cherts, limestones, and 

 shales, Devonian (?) novaculite, Pennsylvanian shales and sandstones, 

 and Permian limestones. A portion of the Cretaceous cover is preserved 

 in the center of the dome on a high ridge which runs from east to west. 

 South of this ridge the Pennsylvanian shales and sandstones on the south 

 side of the Ordovician dome are cut and in part covered or replaced by 

 volcanic breccias and intrusives (figure 2). Volcanic activity is every- 

 where visible in dikes, sills, and laccoliths, the most extensive sill being 

 within 20 to 100 feet of the base of the Cretaceous and extending around 

 a great part of the periphery of the dome, as well as through the inlier in 

 the center. Solitario Peak is an igneous mass, probably a volcanic plug, 

 and an unnamed hill on the north is a laccolith, but no attempt is made 

 to map or describe the other igneous masses. 



The Solitario and the Marathon uplift, 60 miles northeast, are part of 

 an area mountain-built in Permian time and later buried. The broad 

 and gentle uplift of the latter after Cretaceous deposition is believed to 

 have resulted from compression at right angles to the direction of the 

 earlier forces. The very abrupt and symmetrical doming of the Solitario 

 may be accounted for in the same manner or possibly by a laccolithic 

 i]itrusion in basal Paleozoic strata. The Cretaceous cover of the Solitario 



County, Texas, University of Texas, Bulletin 9.3, 1907 ; .T. A. Udden, Notes on the geology 

 of the Glass Mountains ; and C. L. Baker and W. F. Bowman, Geologic exploration of 

 the southeastern Front Range of trans-Pecos, Texas, University of Texas, Bulletin 17.58. 

 1917 ; .1. A. Udden, C. L. Baker, and E. Bose, Review of the geology of Texas, University 

 of Texas, Bulletin 44, 1910 ; R. A. Liddle, The Marathon fold and its influence on petro- 

 leum accumulation, T'niversity of Texas, Bulletin 1847, 1918 ; B. F. Hill, The Terlingua 

 quicksilver deposits, Brewster County, University of Texas, Bulletin 15, 1902 ; T. W. 

 Vaughan, Reconnaissance in the Rio Grande coal fields of Texas, IT. S. Geological Survey, 

 Bulletin 164, 1900 ; .7. A. Udden, The anticlinal theory as applied (o some quicksilver 

 deposits (Terlingua District, Brewster County), University of Texas, Bulletin 1822, 1918. 



