264 CENTRAL MINERAL REGION OF TEXAS. 



of later date than the metamorphics,* and Walcott's equally positive statement 

 that they are all Pre-Potsdam and largely extrusions, -j- we have no published 

 opinion concerning the age of any of the igneous masses with which we are 

 here concerned. J Mr. Hill's diagnosis of the Burnet County granite does 

 not include these earliest outbursts. As Walcott considered all the Pre- 

 Potsdam rocks Post- Archaean (Cambrian at the date of his reconnoissance, 

 but later as Keweenawan), and he regarded as Pre-Potsdam the whole of the 

 great masses of granite "observed in western Burnet and all through Llano 

 County," there must have been some very strong reason for his opinion. 

 This he gives in the paper quoted, in the following words :§ 



At the crossing of the Llano River, on the road from Burnet to Honey Creek Cove, frag- 

 ments of the shales and sandstones of the Llano group may be seen imbedded in the granite; 

 and on Morgan's Creek, Burnet County, the Potsdam rests directly on the granite. 



It need only be said here that both Buckley and Walcott have been partly 

 correct and partly wrong. There are granites of various ages, intrusive and 

 perhaps extrusive; but they are not all later nor are they all earlier than the 

 beginning of the Potsdam epoch. 



It is a very significant fact that the Burnetan gneisses and schists, while 

 they have been more or less affected by eruptions of igneous material in dikes 



An Extrusion is the forcing of igneous matter through the thickness of the strata, and the 

 material of the cross-dike thus produced is regarded as an Extrusive. 



Interbedded igneous masses are such as have been deposited upon one stratum, and then 

 have themselves been covered by later sediments. 



Protrusion and Protrusive are, respectively, nominal and adjective terms for plastic up- 

 heavals beneath strata which have not been penetrated, although often folded and broken 

 by the movement. 



As in our district it is manifestly impossible at this juncture to make these close distinc- 

 tions, the first two terms are here most commonly used. It is to be understood, of course, 

 that igneous masses which are intrusive or extrusive with respect to one set of strata may 

 be protrusive as regards other beds in the same section. 



*Quoted at page 248 of this report. 



\ Quoted at page 249 of this report. 



^Unfortunately, Mr. "Walcott has confounded two or three well marked and unconformable 

 systems under his name of Llano Series, a mistake which could hardly have been avoided in 

 his rapid reconnoissance, for it has taken the writer eight months of very close study to 

 work out this structure. In treating of the typical Llano Series, further on, it will be possible 

 to use Walcott's name, in a restricted sense, as he intended it; but in using the quotation on 

 page 249 of this report it is necessary to bear in mind that its author included all of our so- 

 called Ontarian and Algonkian strata in his Llano series, or group, as he then styled it 

 (evidently with the same intended taxonomic rank as the term series is now given by the 

 International G-eological Congress, and as adopted by the Texas and other surveys.) 



§Paleozoic Rocks of Central Texas. Amer. Jour. Sci. (3rd ser.), vol. XXVIII, Dec, 1884, 

 p. 432. 



