228 SMITH AN J) MENDENHALL — TERTIARY GRANITE 



quite apparent. The rocks gradually lose their original characters, be- 

 coming either schistose or felsitic. At one place the carbonaceous slate 

 lias become a gneissoid rock, the black layers alternating with irregular 

 bands of quartz and feldspar.* This impregnation appears to have been 

 effected by aqueous agencies, and hence the banded rock is not a true 

 injection gneiss ; yet both the added material and its aqueous solvent 

 doubtless had their source in the mass of granite magma intrusive in 

 these sedimentary rocks. 



Along'the contact itself epidote and garnet are found in abundance. The 

 latter mineral occurs both massive and in crystals, and in its massive 

 state the garnet is found in bodies of considerable size. Tourmaline 

 also occurs in the usual association with quartz in the vicinity of the 

 contact. 



Structure 



sedimentary rocks 



The attitude of the sedimentary rocks in itself furnishes additional 

 evidence as to the intrusive character of the granite, and also shows the 

 nature of that intrusion. The slates and sandstones are strongly folded, 

 dips are mostly steep, and sudden changes in both strike and dip are 

 common. Within the area here described the metamorphism often ob- 

 scures the stratification, but a sufficient number of observations could 

 be made to show the general relation of the stratified rocks to the granite 

 mass. The strike in a few places parallels the contact, but more fre- 

 quently it maintains no definite relation. The large apophysis of granite, 

 with the two boss-like enlargements mentioned above, cuts directly 

 across the strike of the slates, which here have a generally vertical dip. 

 In the other places the sedimentary strata present a steep dip toward 

 the adjoining contact. 



NATURE OF INTRUSION 



The facts just cited are sufficient to prove the independence of the 

 structure in the stratified rocks. The intrusion, therefore, was of a bath- 

 olithic rather than laccolith ic nature. An interesting point in connec- 

 tion with Cascade geology would be a determination of the maximum 

 elevation attained by this granite mass at any time during or since its 

 intrusion. At present the granite is known to make up some of the 

 higher peaks of this portion of the range. The amount of cover under 

 which this batholith consolidated can not be more than conjectured from 



* Sixteenth Ann. Rept. U, S. Geological Survey, part 1, p. CG7, 



