72 The American Geologist. February, isos. 
east from the main axis. This dike of i^ranite separates the 
two Red Bed mesas, which dip in opposing directions from it. 
Another dike crosses the country in an east and west direction 
on a hne with the Soda Dam, the waters of which are forced 
to the surface by the dike. The dike is 800 feet wide, and pro- 
trudes up through the Carboniferous and Red Beds strata. 
The rocks of this dike are commonly called granite ; but in 
fact they are eruptive gneiss of the protogine type. The main 
ridge of the Nacimiento mountains is one solid mass of por- 
phyry of the true Rocky Mountain axis (Cope), and extends 
in a north and south direction for many miles. It was once 
covered with sedimentary deposits ; but since the uplift these 
deposits have been removed by erosion so that the entire crest 
is composed of porphyry. The dipping of all the sedimentary 
deposits on the flanks from the main ridge indicates that the 
ridge is intrusive. The successive decrease in the dips of the 
sedimentaries suggests that the whole area has been subjected 
to uplift at difTerent periods. This will be mentioned later. 
The absence of the Carboniferous at the south and also at the 
west indicates that this extension of the Jemez mountains was 
partly at least the result of a faulting on the western side. The 
age of the uplifting seems to be established, because at the 
south of the ridge here the Carboniferous is w^anting, Red 
Beds are deposited against the granite walls, and rest upon a 
granite floor which is a part of the mountain's ridge. As I 
say, this seems to indicate the age of these mountains and that 
it is as early as the beginning of the Carboniferous ; but data 
are wanting, at this time, to explain the apparent relations 
referred to. 
Cochiti Range. — In the Cochiti range from Pelado east to 
Peralta canyon and the mines of the Cochiti gold mining com- 
pany, the core is granite, the same as that of the Nacimiento 
mountains. On top of this is gneiss, over which are volcanic 
rocks and tufas where they have not been removed. From the 
top of the divide east of Pelado along the western side of this 
projection the exposed rock is intrusive porphyry of the labra- 
doryte type. At the south occur the basaltic lava flows which 
have covered the area so that it is difficult to tell what the un- 
derlying rock is. At a point five miles northeast of Valleciete 
Viejo a fault exposes Eocene strata. This fault continues for 
