ORDER OF THE STRATA. 257 



One of the most remarkable is known as the Mammoth ledge, which 

 crosses the valley of Spring- Creek near the original discovery of gold. 

 The stream has cut its channel through the formation, leaving a bluff from 

 20 to 30 feet high on the north side of the valley, exposing a fine section 

 of the ledge. Near the base of the bluff the quartz is intermixed with a 

 large proportion of yellow and brown oxides of iron, which has evidently 

 resulted from the decomposition of iron pyrites. Where the ledge out- 

 crops in the bed of the creek the quartz is found to be full of masses 

 of crystallized pyrites, the water having protected it from oxidation. 

 The ledge is bounded on the west by silicious clay-slates, containing 

 minute garnet crystals, and having a general northwest and southeast strike, 

 and a dip, indistinctly marked, about northeast 60°. The section, com- 

 mencing at the west, is as follows: 



1. Quartzite and ferruginous quartz, interlaminated and mixed with 

 iron ore; width 90 feet 



2. Gray, hard, barren quartzite; 50 feet. 



3. Ferruginous quartz, mixed with quartzite, clay-slate, and iron ore 

 (limonite); 200 feet. 



4. Hard gray quartzite, forming the eastern boundary of the forma- 

 tion. 



This ledge was a study for a mineralogist. It contains undoubted 

 segregated veins of milky quartz, changing in a short distance into cellular, 

 rotten quartz, mixed with limonite iron ore. Some of these masses of 

 vein matter are 10 to j5 feet wide, and continuous for short distances; at 

 other places the quartzite became ferruginous, and merged into brown 

 jasper, or, becoming more pure in its composition, changed to white quartz, 

 as though the silica had replaced, atom by atom, the particles of the original 

 sediment during the metamorphosis of the strata. This view seemed most 

 reasonable in viewing the peculiarities of the banded quartz and black 

 quartzite; each rock, while perfectly distinct, seemed to be the result of 

 the difference in the capacity of the layers of the original sediment to allow 

 the silicated solutions to pass through it. 



There were no evidences of the comb structure so often seen in 

 mineral veins, but everything pointed to segregation as the cause of the 

 17 B H 



