ARCHAEAN GROUP. 267 



also, is the local modification of the later irruptive by contact with the earlier 

 mass through which it has cut; but this is, of course, a less important result. 



IRRUPTIVES OF THE BURNETAN SYSTEM. 



A few well marked features give a distinct facies to the intrusive and ex- 

 trusive dikes and mounds of this system, although our study of them does 

 not warrant the drawing of very sharp lines of demarkation at present. Such 

 generalizations as are defensible are given below: 



1. In the dikes which follow the strikes of the folds there is in most cases 

 the unmistakable appearance of a cooling under pressure. Marked examples 

 of this are the "squeezed" feldspar and the distorted t winnings of quartz 

 crystals at Barringer Hill, Llano County. 



2. The preponderance of quartz in enormous dikes, and as large intrusions 

 in coarse binary granite, and as regular interlaminations in the dense graphic 

 granites, and even in the feldspar itself, is so characteristic that it may be 

 used as an indicator in the search for outcrops of the Burnetan strata. 



3. A class of granites, possibly including both intrusives and extrusives, 

 but more probably restricted to deposits contemporaneous with the deposi- 

 tion of the environing strata, comprises fine grained binary aggregates of 

 feldspar and quartz, rarely containing any considerable portion of mica. 

 These partake closely of the nature of the gneisses of the fundamental acidic 

 Lone Grove Series. 



4. An important, possibly intrusive, basic set of rocks occurs in uncertain 

 relations to the other beds. These may be eruptives of the Burnetan period, 

 and there is not a little support to such an idea, although not enough facts 

 are known to justify such a claim. 



2. FERNANDAS (ONTARIAN?) SYSTEM. 



Wherever an exposure occurs in the trend north 75 degrees west, no rocks 

 have been involved except those already classed in the Burnetan system, but 

 there is another set of folds which includes the same strata, with a considerable 

 addition of superimposed beds of later origin. The name Fernandan* is pro- 

 posed for this system, hitherto unrecognized in this region, on account of the 

 prevalence of the strata in portions of the valley of San Fernando Creek in 



*Mr. "Walcott has already used the more applicable term "Llano" as a series name, in- 

 tended to embrace all the Pre-Potsdam rocks of Central Texas. But as he has made impor- 

 tant discoveries in this region and has reported one prominent unconformity at the base of 

 the Potsdam, it is proper to recognize these contributions to science by the retention of the 

 name Llano as hereinafter applied. It is believed that such terminology does not do vio- 

 lence to Walcott's opinions more than is clearly unavoidable in view of the present writer's 

 later discoveries. 



