MOUNT LINCOLN. ]Q9 



being mostly altered to chlorite. Its groundmass is crystalline and con- 

 tains a considerable development of calcite. Magnetite is also plentiful 

 and has been frequently changed into hydrated oxide of iron. Muscovite 

 is frequently present as an alteration product of plagioclase. The rock as 

 usual contains many fragments of Archean, in this case of rnuscovite-gneiss. 

 The Lincoln Porphyry is like the normal type, but contains few large 

 feldspar crystals. Besides these is a more compact roc*k, apparently a 

 contact product, which in general differs from either rock ; however, some 

 specimens show its probable connection with the Lincoln Porphyry. Its 

 biotite and hornblende are completely changed into chlorite and epidote. 

 The groundmass is very fine and not resolvable into its elements. 



In ascending the regular slope of the ridge westward, as the dip of 

 the formation is slightly steeper than this slope, successively lower beds of 

 quartzite are crossed, and towards its upper end several interbedded sheets 

 of porphyrite. These can be traced along the steep cliff wall overlooking 

 Lincoln amphitheater, and are seen to follow the stratification lines for a 

 considerable distance to the eastward and suddenly bend down into the 

 underlying Archean, thus affording one of the few opportunities of observ- 

 ing the change from a vertical dike into an interbedded mass. Owing to the 

 contrast of the dark color of the porphyrite with the white including 

 quartzite, these bodies can be distinguished from a great distance, and are 

 distinctly visible from the opposite side of the Platte Valley, on the road 

 which leads from Montgomery to the Hoosier pass. 



At its upper or western end, opposite the head of Lincoln amphithea- 

 ter, this eastern spur merges into a basin-shaped valley with debris-covered 

 slopes. On the east face of the northeastern spur, at the head of Lincoln 

 amphitheater, a bare cliff wall affords a section of the lower sedimentary 

 beds and included intrusive sheets, the whole mass much shattered and 

 dislocated. Although time did not admit of the study of these cliff-sections 

 in detail, as was done in the case of others which will be noticed later, the 

 dark color of the intrqsive masses and fragments obtained from the debris 

 show that they are largely of porphyrite, and therefore are probably parts 

 of the sheet already noticed on the east spur. 



