EXDLICH.J VOLCANICS. 197 



scenery it contains is due to the brilliant colors some of the strata 

 belonging to it exhibit . As stated above, the mountains show no 

 arrangement in chains or well-defined systems of ranges, but, in their 

 detail of form, they certainly deserve admiration and attention. At the 

 majority of points, where distinct stratification could be observed in the 

 lawyers composing No. 4, it was seen to be either horizontal, or very 

 nearly so. This fact, together with the unequal hardness of the various 

 strata, produced the result that erosion finally, after other agents i)er- 

 haps had completed the primary separation, was enabled to carve with 

 its skillful hand the most unique mountain forms, beautiful in their 

 symmetry as well as in their detail. The colors of this series are gen- 

 erally dark, with the exception of those shown by one stratum — the 

 "refZ stratum.'''' Onginall}" white, the color has changed into yellow, 

 orange, bright red, and brown. A very thorough impregnation of mi- 

 nute pyrite crystals has produced this change. Upon decomposition of 

 the pyrite,' hydrated sesquioxide of iron is formed, which in turn imparts, 

 according to the quantities in which it is present, the colors above enu- 

 merated. This stratum is found in the lower half of the series. Above 

 it the colors are almost invariably dark, a purplish blue, maroon, and 

 frequently a dark, muddy green. Single bands of lighter rocks occur in 

 the upper members. The thickness of ISTo. 4 is between 3,000 and 4,000 

 feet, containing a by far greater variety of rocks than any one of the 

 lower groups. Without going into detail, it may be well to give a min- 

 eralogical diagnosis of specimens from typical localities. 



As a rule oligoclase takes the place of sanidite, and mica is entirely want- 

 ing. At some points mica was observed, however, in the lower mem- 

 bers. A triclinic feldspar, that may be andesite, occurs associated with 

 the oligoclase in smaller crystals. In some specimens sanidite was 

 found, but it may be regarded as the exception rather than as the rule. 

 The paste is microcrystalliue to compact. 



Above this series of trachytic beds we find in numerous places such 

 rocks that must be and are considered as younger. Frequently occurring, 

 but showing variations in texture and lithological character at almost 

 every x)oiut where it does occur, is the rhyolite. As a rule it may be 

 found su[>erincumbent upon the beds above described, forming caps for 

 some of the highest peaks of the district. As the rocks underlying, the 

 rhyolite shows a well-defined stratification, conformable to that of the 

 older strata. At a number of points its character as such cannot other- 

 wise be determined than by its position and texture, inasmuch as its 

 miueralogical character would scarcely warrant any positive assertions. 

 By the aid of partial or full chemical analysis certainly every doubt 

 can readily be cleared. Near station 10, above 5 miles to the eastward, 

 the most ty])ical occurrence of rhyolite was observed. A deep ravine, 

 bordered on either side by sloping walls several thousand feet in height, 

 contains in its lower portions a quantity of massive basalt reaching 

 upward on the north wall to more than 900 feet above the level of the 

 creek. Resting immediately uj)on this we find a series of rhyolitic beds 

 segregated into narrow strata. It would seem from the position of 

 these two rocks, which can correctly be recognized at a glance, that the 

 latter was really younger than the former. Analogous cases to this 

 have been found, in other portions of the western volcanic regions, but 

 whether they are identical can only be determined by one who has seen 

 a number of them. 



Upon investigation it will be observed that the single narrow strata 

 of rhyolite lying upon the basalt dip toward tlie valley at an angle of 

 60^ to TO'^. There is no evidence that the material could have flown down- 



