PALEOZOIC GROUP. 293 



tured these deposits in places, they probably became extrusive from the 

 Hickory Series. 



The granitic magma must have been more plastic than fluid, because it did 

 not entomb any of the schists, and although its structure now is roughly 

 columnar, or semi-fissile, it appears as if it had cooled somewhat rapidly 

 without great pressure. 



If any further protrusions took place at the close of the Middle Cambrian 

 the action was not pronounced over all the region of the Riley deposition. 

 But there are places where some such factor seems necessary to account for 

 the structure exhibited, as in the valleys of Little Llano and Cold creeks, in 

 Llano County, and perhaps also in the Beaver -Creek region and elsewhere. 

 Of very common occurrence in the country between Smoothing Iron Moun- 

 tain and the King Mountains, particularly along the course of Cold Creek, 

 are what may be appropriately styled "sandstone dykes." These are simply 

 the upturned edges of broken parts of the massive sandstones of the Hickory 

 Series. They are not confined to the north-south trend, but that is a prom- 

 inent course. Sometimes erosion has exposed the granite in juxtaposition 

 with the dykes, and in many places fine springs flow out from the contacts. 

 I have noted many of these projected sandstone ribs and have not found one 

 emerging from rocks of later date than the Cambrian, although their courses 

 are not confined to Pre-Silurian trends. As they do occur cutting through 

 the beds of the Katemcy Series, there was probably a diminished uplift at the 

 close of that epoch; but in our district it was not seriously felt, unless gran- 

 itic outbursts and some faults off to the northwest may yet be traced to this 

 period. From observations made in western Mason County, I am now in- 

 clined to believe that this Post-Cambrian throbbing was the weakened eastern 

 border expression of a prominent dynamic factor off to the west, beyond our 

 district. The northwest course of certain dykes in the Cambrian needs fur- 

 ther investigation, as its importance was not sufficiently appreciated in the 

 field studies. The writer left the field fully convinced that the trend of north 

 36 degrees west involved only Pre-Algonkian rocks, but he is now of the 

 opinion that a reopening of that outlet was partially accomplished at a later 

 period. 



5. SILURIAN* SYSTEM. 



It has generally been believed that the Paleozoic rocks of Central Texas 

 form an unbroken geologic group which accumulated in an open sea cover- 



*The term Silurian is employed in this report as a systemic name equivalent to the 

 Lovjer Silurian of American geologists, it being probable that a new name will soon be 

 adopted for the so-called Upper Silurian. See Dr. J. D. Dana's remarks upon this subject in 

 Bulletin Geological Society of America, vol. I, February, 1890, page 40. 



