omui. NEN 
NOTE 
IN 
EXPLANATION OF THE MAP AND SECTION. 
Tue Geological Map which I have prepared to accompany this Report exhibits the line sur- 
veyed by Captain Pope, with its relations to the Mississippi river and the shore of the Gulf. 
The scale is the same with that of the General Map of the United States Territories now in 
preparation in the Office of the Pacific Railroad Explorations and Surveys. The ranges of 
mountains are indicated by a broken line, instead of the ordinary shading used by topographers. 
The width of the map has been made to correspond with the quarto page, so that it need 
not be folded in more than one direction. 
The section is drawn to an equal scale for heights and distances, or half an inch to the mile. 
The direction of the section corresponds with the line of survey as marked on the map, except 
the extreme western end, which follows the parallel of 32° of latitude from the Rio Grande 
river to the Hueco mountains, where they are crossed by the trail. 
The western portion of the section is nearly transverse to the trend of the mountains and the 
strata, and therefore gives a true representation of the strata, so far as they are known. The 
eastern portion, however, is believed to be nearly, or approximately, parallel with the trend of 
the Carboniferous rocks near Fort Belknap, and the direction of their dip cannot, therefore, be 
shown with any accuracy. In coloring this section, I have distinguished between the Creta- 
ceous rocks of the surface of the Llano, and the subjacent red marls, clays, and sandstones 
which contain the gypsum. They are also distinct from the Carboniferous, although it is my 
opinion that further investigations will connect these strata with one of the two formations, 
probably with the former, if they are found to be unconformable in the Guadalupe Pass and at 
other places. In either case, the separate coloring is desirable, as there is sufficient difference 
of mineral characters to authorize a division for convenience of description, even if the strata 
are not separable by a difference in their organic remains. 
Tertiary strata are not represented upon the Llano and the plains beyond, for the reason 
that there is no evidence of their presence. It is probable, however, that they occur either on 
or near the line of survey, but they may have been entirely removed by denudation, The 
detrial accumulations, or the wash and debris from the mountains, are also unrepresented. It is 
probable that they are quite thick about the mountains, and especially at the base of the Organ 
and of the Hueco mountains; and they may extend over a great part of the plains between the 
ranges, so as to hide the older formations—the Tertiary or Cretaceous—from view. In repre- 
senting the upper or light-colored strata of the Llano—and, indeed, all the formations—it was 
found desirable to distort them in thickness, so that their relations would be more apparent 
to the eye. ` : 
It is, of course, impossible to represent accurately the position and thickness of strata under- 
lying the surface, and which are not exposed at short intervals, by sections. The section, and 
especially that part representing the Carboniferous rocks below the surface, is to a great extent 
ideal, and is not presented as accurate, but will, it is hoped, serve the purpose of giving a 
