256 CENTRAL MINERAL REGION OF TEXAS. 



difference in the drainage features of the Colorado River basin in Burnet and 

 Llano counties from what is exhibited elsewhere in that valley and in other 

 Texas hydrographic basins. The map sheets of the United States Geological 

 Survey published in 1887, although imperfect, are accurate enough for this 

 purpose in this region. Referring to the Burnet and Llano sheets, a little 

 examination will reveal certain trends in the arrangement of mountain peaks 

 and in the courses of streams which are different from those recorded in other 

 areas. In other words, there is an area in western Burnet County, and in east- 

 ern Llano County especially, where more of these trends can be found than in 

 any other section, within the limits of this review at least. As we pass out- 

 wards, upon all sides from this tract the stream courses and the variations in 

 the trends of relief forms become less numerous, until at a considerable dis- 

 tance only one or two such bearings can be detected as prominent topographic 

 features. More attentive study of the maps will make apparent the fact that 

 there is one well marked line which is almost exclusively confined to limited 

 portions of the Colorado drainage between the watersheds north and south of 

 the Llano River between latitude 30 degrees 40 minutes and latitude 30 de- 

 grees 50 minutes in Burnet County. This particular set of trends bears nearly 

 north 75 degrees west,* and, if it be in itself a primary element of structure, 

 it probably marks the course of the oldest forms of continental relief in Cen- 

 tral Texas. There is at least a possibility that the rocks now exposed in the 

 nucleal area are part of a general trend more nearly north-south, and there- 

 fore of more importance in the matter of continent building than the present 

 trend of the exposure would imply. But the bearing given as partially the 

 result of erosion is also a marked structural feature, being the real strike of 

 a series of folds which seem to be stratigraphic, and not merely foliation 

 planes. An element of uncertainty regarding the real original trend of these 

 basal gneisses arises from the occurrence of a much later line of upheaval, 

 which bears along a line within 15 degrees of this course, and which has, in 

 places, involved the Archaean rocks as well as those of more recent eras. 

 But, while it is probable that the later upheaval has been coerced in part into 

 the ancient Archaean trend, owing to that being the line of least resistance, 

 the writer has recorded a number of instances where both trends can be 

 clearly made out in the same area. For this reason it is tentatively an- 

 nounced that the fundamental gneisses originally occupied a lens-shaped area 

 striking north 75 degrees west by south 75 degrees east through the region 

 previously defined. 



The reason for suggesting a possible north-south trend to the Archaean 

 protaxis is the persistence of salient points along such a course, a feature no 



*N. 84 deg. W. magnetic, 1889. Yariation 9 deg. 15 min. E. Corrected, N. 74 deg. 45 

 min. W. 



