312 CENTRAL MINERAL REGION OF TEXAS. 



the results are comparatively slight in the way of upturning of the strata, 

 but the two disturbances since the Paleozoic Era have left enduring monu- 

 ments in the shape of highly tilted rocks. Such slight unconformities as I 

 have observed among the marginal Post- Silurian deposits, aside from the 

 Cretaceous, are wholly due to overlap, and this statement is, I believe, con- 

 firmed by the reports of Messrs. Durable, Tarr, and Cummins, who have 

 given the subject more careful study in the field. There is no warrant, 

 therefore, for referring either of the later upheavals to a time earlier than the 

 close of the Carboniferous period. 



A. THE PRE-CRETACEOUS MOVEMENT. 



In the outer area of the Carboniferous, as well as in the earlier rocks, there 

 are numerous fractures and faults trending almost due east and west.* Some- 

 what similar breaks may also occur in parts of the Cretaceous, but a different 

 explanation must be sought for these, because it is demonstrable that compara- 

 tively few of the Carboniferous joints extend upward into the later deposits. 

 In the region occupied bf the crystallines and the more ancient sediments 

 the manifestation of this latitudinal uplift is extensive and expressive. We 

 do not yet know all its geography, but it is very significant that the most im- 

 portant structural results in our district were confined to the drainage of 

 the Llano River and Big Sandy Creek. The igneous action was wholly 

 south of the Llano River, so far as can be judged from the observations, 

 unless some provisional Archaean west of Mason, on the Menard road, shall 

 prove to belong to this category. A very stubborn problem is presented 

 in this locality, and I may not have solved it correctly, although some very 

 accurate topographic and geologic work has been done upon it. The prob- 

 ability is that every one of the seven or eight trends of uplift is included in 

 the structure, although only three were clearly recognized in studying the 

 outcrops. 



Not far south of this the combinations are different, and as is usual where 

 very comprehensive sections are exposed, there is liability to err in interpret- 

 ing the structure unless the key has been worked out in less complicated 

 areas. Without the knowledge of the meaning of each trend which has 

 since been gained, it is hardly probable that any one could unravel the intri- 

 cate network. It is very possible that the Post-Paleozoic upheaval followed the 

 old Burnetan trend more closely in some places than in others, in which case a 

 very good acquaintance with the textural habit of each of the two granites will 

 be necessary in order to distinguish them. The fact remains, however, that the 

 Post- Silurian granite is actually cut through in places by dikes of the material 



*North 80 degrees east, magnetic; declination, 9 degrees 15 minutes east; corrected, north 

 89 degrees 15 minutes east, 



