282 



CENTRAL MINERAL REGION OF TEXAS. 



by the writer in a former publication, where the whole subject is more fully 

 treated, with illustrations from another field.* 



Finally the overlying, much crowded, and continually thickening capping 

 of strata, although undergoing gradual metamorphism and becoming much 

 folded, would only afford vents for either basic or acidic lavas in places 

 where lines of weakness had been induced; and the granites could but rarely 

 avail themselves of these. 



The foregoing theoretical considerations are based wholly on the record of 

 the rocks of our area, and they are borne out in fact in every important par- 

 ticular by all the observed igneous occurrences. 



At the close of the Eparchsean Era we also part, so far as we now know, 

 with the intrusive granites as a prominent structural feature. By this I mean 

 that the compressed interstitial granites of the Burnetan, and the compacted 

 quartzose granite dykes of the Fernandan, as well as the Texan type of 

 tough feldspathic granite with moderate sized crystals, are not found asso- 

 ciated with the later rocks, nor are their characteristic modes of occurrence 

 repeated in more recent periods: 



These generalizations will probably admit of better application when the 

 results of microscopic studies are available. At present they may be sched- 

 uled as below. 



Plutonism. 



Vulcanism. 



Strata. 



Time. 



Protrusion. 



Protrusion. 

 Extrusion. 



Protrusion. 



Extrusion. 



Intrusion. 



Protrusion. 

 Extrusion. 

 Intrusion. 

 Extravasation. 



Doubtful. 



ob 



Basic. .2 



a 



Acidic. -o 



Basic. $ 



1 

 Acidic. ™ 



is 



o 



Basic. | 

 Acidic. <g 



Cambrian. 



Texan. 



Fernandan. 



Burnetan. 



[• PALEOZOIC. 

 1 EPARCELEAN. 



► ARCHAEAN. 

 > 



Thus prepared, the great time gap which is represented by the denudation 

 of the Texan folds before the deposition of the Cambrian strata will be found 

 to be bridged over by leaves of history and by elements of structure hitherto 

 unreported from Central Texas. 



*The Geology and Vein Structure of Southwest Colorado. By Theo. B. Com stock. 

 Transactions American Institute Mining Engineers, vol. — , 1886, pp. — . Also published 

 separately in pamphlet form. 



