292 



CENTRAL MINERAL REGION OF TEXAS. 







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IKEUPTIVES OF THE CAMBRIAN PERIOD. 



Heretofore we have only been able to 

 study the irruptives as concomitants of 

 the strata in which they occur, making 

 determinations of their time relations to 

 each other, and offering hypothetical 

 opinions for the most part as to their 

 geologic ages. Perhaps we shall yet be 

 able to clear up many doubtful points 

 concerning the ancient dynamic history ; 

 but there are manifest obstacles in the 

 way of deciding closely whether an ig- 

 neous ejection in the schists may have 

 been intrusive, extrusive, or only pro- 

 trusive. Fortunately not much is now 

 dependent upon these minor issues in 

 our stratigraphy, although a full knowl- 

 edge of the character of the special plu- 

 tonic and volcanic products may event- 

 ually be essential to the solution of im- 

 portant problems. 



In the Paleozoic rocks there are less 

 confusing contacts with the granites, and 

 I do not know of any basic lava flows 

 nor of any other than protrusive granite 

 masses later than the Eparchsean. There 

 are granites which now extrude from 

 the Pre-Paleozoic rocks, but these can 

 always be traced into such relations with 

 Paleozoic Beds that their Post-Texan age 

 can not be doubted. Reference has been 

 made to probable extrusions of granite 

 along meridianal belts, accompanied by 

 subsidences of intervening areas, at the 

 close of the Lower Cambrian. The 

 course of these uplifts had been marked 

 out previously by much denuded and 

 weakened folds in the Texan strata. The 

 metamorphosing effect of the outbursts 

 was not marked, although, as they frac- 



