REMARKS ON THE MAP AND SECTION. 
175 
In those portions of the section which do not follow the line of the trail, and which represent 
a region over which no measurements were made, the distances ot the important points were 
determined by measurement upon the maps. Some of these measurements were made upon the 
manuscript general map of Lieutenant Whipple’s report, and others on the general map of the 
office and on the geological map. The scale of these maps is so small, compared with that 
of the section, that slight inaccuracies in measuring, or in the maps, become very apparent 
when placed on the section. If, therefore, the distances on the section he compared with those 
on the geological map, many unimportant discrepancies, arising from these several sources of 
error, may doubtless he found ; hut they are believed to he, in general, of little consequence to 
the accuracy of the geological representation for which the section is intended. In many cases, 
also, it will be found that the distances between the camps, where the section is said to be along 
the line of the trail, do not correspond exactly with those obtained by the viameter measure¬ 
ments, an.d given in the tables of the report. This discrepancy arises from the disregard of the 
short and circuitous winding of the road, which increases the distance between the camps, with¬ 
out adding any new features to the geology. Indeed, in some instances the distance between 
the camps was taken by the dividers from the map and transferred to the section, without 
regard to the distance measured by the viameter and recorded in the report. 
The level of the trail is shown on the section by a straight and continuous fine line, and the 
elevations and bluffs are generally sketched above it, as may be seen along the representa¬ 
tion of the Llano, where the bluffs, or escarpments, rise above the general line of profile. 
In this case the trail line forms a convenient line of separation between the colors representing 
the cretaceous strata and the red sandstone containing gypsum below, and is made use of 
for that purpose. When, however, the trail leaves the valleys and ascends to the surface of 
the plateaux, the upper line of course becomes the line'of trail, and the lino below, dividing 
the cretaceous, Or other formations, from those next below, is not continuous but broken. All 
the lines dividing one formation from another are continuous, when above the trail, but on 
descending below that line—below the line of observation—they are broken, dotted, and 
show that the position of the formations is not accurately known. 
As the scale adopted did not permit altitudes of less than 500 feet to be very distinctly repre¬ 
sented, it became necessary to slightly distort the altitudes of portions of the line in order to 
make them distinctly visible. The alluvial deposites of the Mississippi could hardly be 
separated by the eye from the base-line of the profile if they were drawn according to the 
scale. They have, therefore, been distorted. So, also, the hills at Little Rock, which are 
probably less than 300 feet in elevation, are sketched as if they were 600 feet. The bluffs 
also along the valley of the Colorado Chiquito, and in general the depths of the canons and 
channels of the streams, are slightly overdrawn. These departures from the scale are, however, 
of little importance, and do not materially affect the truthfulness of the section. 
Of course, the greater part of the representation of those portions of the strata which are 
below the surface is ideal, and is based only upon the indications presented at the outcrops 
and by the form of the surface. The representations might have been confined to the out¬ 
crops which were seen above the trail, leaving the space between the profile-line and the base¬ 
line blank ; but although this would be sufficient for the geologist, it would fail to convey 
an idea of the geological structure of the line to the general reader. The probable position 
of the underlying rocks is shown ; that their position and relation to each other is accurately 
displayed is not claimed. It is most probable, as has already been explained, that the carbon¬ 
iferous formations are much more bent and flexed than appears in the parts of the section 
east of the horizontal outcrops at the Aztec and Aquarius mountains. 
