390 THE JOURNAL OF GEOLOGY. 
tioned pebbles may be called augite-syenite. A partial analysis 
of one of them (No. 30 Amador county) by Dr. Stokes of the 
United States Geological Survey, gave the following results: 
No. 30 165 
Amador. S.N 
SiO, ° Ss -45 2 ° - 55.04 
MeOn - - Mow - - 3.41 
Kk,O - - at, Bailes) - - - 1.41 
N,O - - e € 167/33 - - 4.27 
A rock in place closely resembling these pebbles has been 
found near Nevada City, by W. Lindgren, and is shown on his 
Grass Valley sheet in the Nevada City folio. To the north of 
Tehuantepec Valley in the area of the Downieville atlas sheet is 
a large mass of granitoid rock that has very nearly the same 
mineral composition and structure as the pebble above described. 
An analysis by Dr. Stokes of a specimen from this area is given 
above (No. 165 S.N.). This rock is composed of’ plagioclase, 
augite, rhombic pyroxene, iron ore, and brown mica enclosed in 
later unstriated feldspar that appears to be orthoclase, although 
the chemical analysis does not show enough potassa for much 
orthoclase to be present. The augite is plainly later than the 
plagioclases in places indicating a tendency to ophitic structure. 
There is a little quartz in the rock. All the constituents are 
fresh. 
In the area of the Placerville sheet in southern Eldorado 
county, in the same belt of Carboniferous rocks noted above as 
containing conglomerate, there are numerous areas of igneous 
material. One of these, forming the hill known as Big Sugar 
Loaf, is a boss of porphyrite containing quartz and hornblende 
phenocrysts, and extending from this mass north across Slate 
Creek is a narrow dike containing abundant primary brown horn- 
blende needles. This porphyrite boss is believed to represent an 
eruption of Carboniferous time. About three and a half miles 
north of Big Sugar Loaf is a hornblende-porphyrite area, and 
thin sections of the rock show a devitrified groundmass exhibit- 
ing flow structure. It is possible, however, to regard these 
masses as intrusive and younger than the enclosing slates, but 
