268 CENTRAL MINERAL REGION OF TEXAS. 



Llano County. These beds strike about north 36 degrees west,* a trend which 

 is well represented upon the maps of Llano and Mason counties as the general 

 courses of many of the streams flowing into the Llano River from the north. 

 There is abundant evidence that the dynamic event which is recorded in this 

 structural feature was not a mere local phase of history; for its effects are to 

 be seen in the exposed Burnetan rocks in the dykes and master joints which 

 traverse them, and the later systems have, in a measure, indicated continua- 

 tions of the uplift by their shore lines and by the distribution of certain seg- 

 regated minerals. 



An exceedingly interesting item is the sparing occurrence of the folds of 

 the Fernandan System upon the east side of the Colorado River. East of a 

 line, approximately in the strike of the system, extending from near the head 

 of Little Llano Creek southeastward beyond the great bend of the Colorado 

 below the mouth of Sandy Creek, no Fernandan rocks have been observed, 

 excepting two small outcrops of the highest beds in Hoover Valley and near 

 Nigger Head Peak. This illy-defined border seems to have been about the 

 course of a shore line of the Post-Burnetan sea, and the present topography 

 is, in part, influenced by that ancient feature, although the exposures of the 

 extreme ends of the tract now have a difference of level of more than two 

 hundred feet. This last fact has a most important bearing upon conclusions 

 to be given later, which are entirely new, and which investigators in neigh- 

 boring fields will be disposed to accept grudgingly without every scrap of 

 the evidence which the author has collected. The extent and outline of the 

 coast of the sea in the Fernandan Period can not be made out, owing to the 

 faults and igneous intrusions of later date, which have dropped outlying por- 

 tions of the area. But the contact of these rocks with the Burnetan System 

 can be fairly traced across most of the country between Lockhart and Pack- 

 saddle mountains. The character of the erosion within these limits, although 

 much modified by more recent dynamic results, has undoubtedly been af- 

 fected materially by the topography and geognosy of the Burnetan and the 

 Fernandan periods. 



The exposures of the Fernandan Beds are more extensive than the out- 

 er oppings of any of the Pre- Cambrian systems. No western shore line has 

 been detected in Central Texas. The contacts with later strata upon the 

 western flank of the uncovered area all indicate a much greater extent be- 

 yond the limit of our field. There is a possibility that islands of the Burne- 

 tan system, or even an extended land area of that time, may have existed in 

 the Fernandan sea in western Llano County and eastern Mason County. But 

 from the greater development of the northwest trend in the mountain chains 



*North 45 deg. west, magnetic, 1889; variation, 9 deg. 15 min. east; corrected, north 35 

 deg. 45 min. west. 



