MOUNT LINCOLN. Ill 



The tvpical Lincoln Porphyry, as fonnd on the summit of Mount Lin- 

 coln itself, is characterized b}- large orthoclase crystals, which sometimes 

 reach two inches in length, of pinkish color, generally Carlsbad twins, and 

 often so fresh and glassy in appearance as to remind one of the sanidine 

 crystals of more recent rocks. There are five or six large crystals as a 

 rule in an ordinary hand specimen. The smaller feldspars are white, and 

 a large number show distinct striae. Quartz is very abundant and rela- 

 tively large, in round grains, often of pinkish hue, and showing more or 

 less plainly the fsxces of dihexahedral crystals. Biotite in darker or lighter 

 green leaves, according to its condition of decomposition, is quite conspic- 

 uous in the rock. A few specks of specular iron are sparingly scattered 

 through the rock. The groundmass is light green or pinkish and is quan- 

 titatively quite subordinate to the crystalline element. Under the micro- 

 scope it is seen to be fully crystalline. 



Such is the typical Lincoln Porph^'ry, which projects in lofty columns 

 from the summit of the mountain in a sharp apex which overtops all the 

 surrounding peaks. Owing to its exposed situation it attracts the storm 

 clouds from all the regions around, and even in inidsummer scarcely a day 

 passes without a slight fall of snow or hail on the summit. The very 

 topmost rocks show traces of discharges of the electric fluid in the forma- 

 tion of fulgurite, which encircles the little holes it has bored into the rocks. 

 Around the base of this summit mass of porphyry its contact with the sed- 

 imentary rocks is obscured by debris, the few outcrops that are seen being- 

 composed of rocks so much altered that their original character cannot be 

 determined. 



Cameron amphitheater. Qn the Stecp SOUtll faCO of Liucolu, a skctcll of 



which is shown in Plate XI, a careful study was made of the various erup- 

 tive masses. The Lincoln Porphyry of the eastern edge of the sunmiit is 

 of a nuich darker color than the normal rock and contains few or none of 

 the larger feldspar crystals. It is so much decomposed that only in the 

 center of large blocks is the original grayish color preserved ; but the round 

 quartz grains are distinct throughout. Tlie Blue Limestone, which here 

 seems to have a brecciated structure, can be traced as a horizontal line 

 across the face of the cliff, from the Present Help mine on the west to the 



