CLASSIFICATION OF PRE-PALEOZOIC IGNEOUS ROCKS. 281 



basic layers are proofs of a somewhat vacillating sea border, and other indi- 

 cations of this appear in the lack of constancy in the vertical sections. 



Irruptive rocks occurring now in this basal series of the Texan System 

 may have been eruptive, intrusive, or extrusive, or of all three kinds, and it 

 will not be a simple task to trace them all to their sources. 



In the Llano Series the traces of interbedded basic eruptives towards the 

 close of the epoch are rather more definite. This is especially true in the 

 western exposures, although thinner interpolations, perhaps less numerous, 

 occur as far east as Packsaddle Mountain. 



The Packsaddle Epoch does not appear from present information to have 

 been a time of volcanic activity in our region. The sea must have deepened 

 considerably and gradually to have allowed the deposition of the shales and 

 limestones. There does not seem to have been any landward encroachment 

 of the sea, but rather a contrary movement in a small way. This foreshad- 

 owing of later orographic movements of much importance was, as far as we 

 now know, unaccompanied by any extensive eruptions, although some obser- 

 vations leave a suspicion that intrusions of an acidic (granitic) magma were 

 injected before the upheaval at the close of the Texan Period. 



"When the granites of this system are considered, a very difficult problem 

 is presented, and one which may be more clearly stated, if not more easily 

 solved, when the facts connected with the later history have been announced. 

 It is proper to state here, however, that after the Eparchaean there ensued an 

 order of conditions which opened a new arch-era in geologic times in this 

 region. 



CLASSIFICATION OP PRE-PALEOZOIC IGNEOUS ROCKS. 



All through the great systems of the vast Pre-Paleozoic Eon eruptions of 

 the basic igneous rocks must have been frequent and abundant. Granitic 

 injections, from their fluidity, easily reached the surface in the earlier periods 

 and furnished the material of the fundamental gneisses and schists, at least 

 in part. 



Gradually, as the later deposits accumulated, the intrusions and extrusions 

 met greater resistances and smaller masses overcame the tension of the 

 superincumbent strata. The highly acidic character of the suspended super- 

 structure rendered the melting process less and less effective, and perhaps be- 

 fore the close of the Archaean the intrusion of granitic masses had practically 

 ceased. Not so, however, with the basic igneous products, which at the same 

 temperature were more fluid, and which would find escape at points not ac- 

 cessible to the higher lying and less dense acidic magma. This is explained 



