332 G. D. LOUDERBACK — STRUCTURE OF THE HUMBOLDT REGION 



side with respect to the other, with concurrent deformation of the topog- 

 raphy, while streams that have gradually worked into such a path on 

 account of the disparity in hardness between the two walls or because of 

 the weakness of the crushed zone, just as streams gradually establish 

 flow lines along soft strata, may be called subsequent. This distinction 

 can not, however, always be made in practice. 



In this sense the fault gulches just described are believed to be conse- 

 quent, for (1) the rest of the drainage above and below them is of an 

 unadjusted juvenile character, and an adjusted development in the midst 

 of it and so sharply separated would hardly be expected ; (2) the rocks 

 of the two walls are the same lithologically, the fault being determined 

 by discordance of attitude, and (3) the fault is parallel to the system of 

 normal faults which on the east slope have produced many consequent 

 stream valleys. 



The fault vallej^s on the east slope are quite abundant. As described 

 above * the faults can be determined easily stratigraphically on account 

 of the simplicity of the rock series. Several are shown in the section, 

 and also in plates 17 and 18. The chief interest in these faults is that 

 they show that the volcanic series has been in many places dislocated 

 by normal faults ; that the mountain block is broken into secondary 

 blocks by minor faults, which state of affairs b}^ analogy we expect in 

 the valley blocks; and that extension, not compression, has been the 

 dominant physical condition in the deformation of the volcanic series. 

 The fault valleys in the volcanic series also show us that consequent 

 drainage lines may have properties commonly supposed to be character- 

 istic of subsequent stream channels in particular, extension along the 

 strike and in soft strata, and extension parallel to the axis of uplift. 

 Such characters, therefore, are not sufficient tests of subsequent streams 

 unless recent faults have been proved not to exist. 



The summit basalt breaks off in a precipitous cliff facing westward, 

 down which the slope descends b}^ a series of slips with planes close to- 

 gether, thus covering the cliff face with basalt (not simply talus) and 

 absolutely obscuring the underlying tuff. This obscuration of the tuff 

 has been noted at many places, and its presence has sometimes been 

 difficult or even impossible to ascertain except by following the exposure 

 a mile or more. 



A mass of diorite is exposed near the summit and continues down the 

 east side of the range. Its distribution is unsymmetrical with respect 

 to the range crest, as it is mainly on the east side. The eastward dip 

 of the rocks not far west of this diorite (60 to 65 degrees), which would 



* See pages 305 and 314. 



