70 The American Geologist. February, 1903. 
The trachyte-rhyolyte lavas underlie the tufa in the vicinity of 
Pelade and the Valle Grande country. Their entire thickness 
Was not ascertained, though 400 feet of these lavas, in vertical 
section, were measured to the east of the Jeniez river in the 
vicinity of the Jemez sawmill. 
The sedimentary rocks are the country rocks of nearly the 
entire region, where not covered with lava. They were laid 
down in the seas and lakes that surrounded the islands which 
now form the high mountains of San Dia and Jemez. These 
deposits date back well into the Carboniferous, and continue 
almost without break to recent times. 
General sections. — In order to give an idea of the general 
stratigraphy of the region under consideration, three general 
sections will be given : ( i ) extending from Jemez peak in the 
Nacimiento mountains to the fault at the foot of the San 
Dias; (2) from the same peak south to the Rio Puerco; (3) 
from La Vantana on the Rio Puerco to Tejon, north of the 
San Dia mountains. 
In passing from Jemez peak along the first section, we tra- 
verse four miles of the Jemez-Nacimiento mountain core of 
granite and red orthoclase ; then over Carboniferous, Permian, 
and Red Beds. The three latter are conformable and dip at a 
steep angle toward the southeast away from the mountains. 
Beyond the Red Beds is a small patch of Pliocene one-half 
mile in width, with strata dipping gently to the north. Pleis- 
tocene is then encountered, and continues with strata nearly 
horizontal for about two miles, when Pliestocene recurs with 
strata dipping slightly to the northwest. These strata form 
the east bank of the Jemez river at San Isidro. After crossing 
this deposit we again come upon the horizontal beds of the 
Pleistocene of the Jemez river on the east side of the big bend. 
After crossing this flood plain, about two miles in width. Cre- 
taceous strata crop out. These rocks dip toward the south- 
east. On climbing up the divide, we enter upon the Zia marls 
which continue to the Rio Grande, a distance of twenty-eight 
miles. The alluvium of the Rio Grande valley extends from 
the west bank of this river to the foot of the San Dias, a dis- 
tance of about twenty miles. 
The second section from Jemez peak south to the Rio Pu- 
erco presents the following surface exposures : three miles of 
granite, seven miles of Red Bed strata, dipping at a great 
