UNDGHBN.] PLACERS OF THE BOISE RIDGE. 719 



NEOCENE PLACERS. 



The shore and old gulch gravels resting on granite in the early Neo- 

 cene Payette formation carry a little gold at many places for instance, 

 in several gulches about 2 miles eastward from Boise, north and south of 

 the Idaho City stage road. Similar old placer deposits are found in 

 the Payette formation at Church's, in Marsh Valley, and the old grav- 

 els at Johnson's, 1 mile southwest of Marsh, contain some gold which 

 has been concentrated in the gulches and washed by the hydraulic 

 process. 



Gold-bearing gravels of late Neocene (Pliocene) age are found below 

 the remnants of the several basalt flows on both sides of Boise River, 

 those in Moore Creek having already been mentioned. The top of the 

 lowest flow, which is probably the oldest, lies at the ' ' New York House," 

 10 miles southwest of Boise, at the level of the river, and is not visible 

 farther west. Eastward it rises slowly, and near the mouth of Moore 

 Creek is 40 feet above the river (in August). Below this flow, which 

 is about 20 feet thick, lie 2 to 10 feet of coarse, heavy gravel, resting 

 on granite. This gravel is in places rich in coarse gold, part of which 

 probably conies from seams and small veins in the surrounding gran- 

 ite. There are only a few exposures of this low flow below the mouth 

 of Moore Creek, and it is reported that only one or two are known 

 above. The flow came down the south fork of the Boise. At low 

 water this gravel below the basalt has been mined with profit at sev- 

 eral places, notably at the Holy Terror mine, 2 miles below the mouth 

 of Moore Creek, and at Tarents, 2 miles farther down. There is only 

 a limited amount of this gravel below the lower flow. The two other 

 flows, 30 to 60 feet thick, are at elevations of 120 and 300 feet above 

 the river. Underlying both of them, wherever they are preserved, 

 hanging along the banks of the canyon, are heavy masses of late Neo- 

 cene gravels, 20 feet or more in thickness. This gravel contains some 

 gold throughout, and though much of it is fine, it may in places be 

 found rich enough for the hydraulic process wherever water can be 

 brought to it. In 1896 an attempt was made at the mouth of the 

 canyon, 8 miles southeast of Boise, on the northeast bank of the river, 

 to mine the heavy mass of gravel and sand here underlying the basalt 

 flow, and should this attempt be successful there are probably many 

 other similar deposits a little farther up the river which could be 

 worked in the same manner. 



