262 CENTRAL MINERAL REGION OF TEXAS. 



pits (pot holes), knobs, and crusts is very noticeable. These irregularities are 

 due to differences in hardness, and possibly in density — the more durable 

 parts having somewhat of a concretionary character as developed by decay. 

 The lamination or foliation of the gneisses, not always apparent before disin- 

 tegration, tends to curve about the hard kernels.* Alterations have also 

 taken place in many of the porphyritic and schistose rocks, some of which 

 are very interesting. Kaolinization is very common. A fine example of this 

 may be seen two and one-half miles south of Valley Spring, in the bed of 

 Johnson Creek where it turns suddenly eastward into this strike. Quantities 

 of hydrated brown and yellow encrustations, often in the form of the original 

 gadolinite and accompanying minerals, are constantly taken from the Barrin- 

 ger hill, on the Colorado River, in Llano County, north of Long Mountain. 

 These contain large proportions of cerium, thorium, yttrium and associated 

 metals. An instructive example of the alteration of amphibole to biotite 

 upon a large scale is exhibited in the north gap of the King Mountains, at 

 the head of a branch of San Fernando Creek, and numerous other illustra- 

 tions might be given. » 



2. While the structural details of the Burnetan System have been but 

 partly elucidated, there is no doubt that: 



(a) The exposed plateaus and the isolated fragments of the original islands 

 represent in the area now exposed at least two broad elevated belts of the 

 basal gneisses of the Lone Grove Series. 



(b) One median basin and the exposed parts of two exterior troughs now 

 contain the greatly denuded remnants of the basic and acidic Long Mountain 

 and Bodeville Series. 



(c) It is impossible as yet to state whether any unconformity exists be- 

 tween the probably earlier Long Mountain Series and the overlying(?) schists 

 of the Bodeville Series. 



3. A fact of the greatest moment, and one which has escaped the attention 

 of previous observers, is the existence of a great unconformity between the 

 rocks of the Burnetan System and those of later date. 



4. As will be shown in another part of this report, there are enormous 

 quantities of undoubtedly Post- Burnetan strata whose composition indicates 

 clearly an origin from the degradation of the Burnetan System. From 

 which follows a suspicion that the three rock series now under discussion 

 were deposited conformably inter se, and that their folding as one system 

 took place at the close of what may have been the Laurentian Period, at any 

 rate after the deposition of the Bodeville Series. 



5. This last conclusion is somewhat fortified, or at least it is not antago- 



*This process of weathering is similar to what Mr. Lawson describes at page 24 of the 

 report quoted in the last preceding foot note. 



