4 GUIDEBOOK OF THE WESTERN UNITED STATES 
their deposition. The wind has also played some part in the accumu- 
lation of sand, and much detrital material has been moved by glaciers. 
The character of the rocks and the order of the sedimentary succession 
vary in different regions; thus a system may be fully represented in 
one district and be entirely or partly absent in another, owing to lack 
of deposition or to removal by erosion in an interval of uplift. In 
some places a portion of geologic time may be represented by limestone 
alone, while in others the same portion is represented by shale, sand- 
stone, or volcanic rocks. Fossil shells, bones, and other parts of 
they are the key to geologic problems, such as the structure and order 
of formations, that in many places could not be solved without them. 
The total thickness of the sedimentary formations is many thousand 
feet, and the time represented is hundreds of millions of years. 
Note.—For the convenience of the traveler the sheets of the route map in this 
bulletin are so arranged that they can be unfolded one by one and kept in view 
while reading the related text. The contour lines, in brown, represent elevation 
above sea level. Each line indicates the path that would be taken by one who 
walked over the country by a course always at the same level, curving in and out 
with the irregularities of the land surface. The lines are drawn at the vertical 
distances apart (‘‘contour interval”’) stated on each map; where these lines are 
close together they indicate a steep slope; where they are far apart, a gentle slope 
orplain. Most of the contour lines have been compiled from detailed topographic 
maps published by the United States Geological Survey, the names of which are 
given in the southwest corner of each. In some areas the contour lines are taken 
from surveys by the Engineer Corps, U. S. Army, and the Los Angeles Depart- 
ment of Water and Power, and from reconnaissance by the author. A reference 
to each map is made in the text at the place where it should be unfolded. The 
areas covered by these sheets are indicated on Plate 2, and a list of the sheets 
and the other illustrations is given on pages 11I—vil. 
res given on mileposts in Louisiana indicate miles from Algiers, a Southern 
Pacific freight terminal on the south bank of the Mississippi River opposite New 
ime they are about 1% miles less than the distance from the Union Station, 
West of Houston the mileposts give distances from Harrisburg, 
an Rae terminal off the present main line about 3 miles south of Houston station. 
West of El Paso the mileposts give ‘distances from San Francisco. As a rule the 
mileage given on the posts does not allow for differences due to subsequent 
eaeelerscoe.! or Provencte sige ae of — eet _ Most of rena ethan at stations given in 
compan but others are derived 
from | precise levels of the United “it Coast and Geodetic Survey or the United 
States Geological Survey. 
The statistics given in t idebook tal official Government 
sources, such as the United States Census and the United States Bureau of Mines. 
stated or not. Authors’ names cited in parentheses refer to the bibliography on 
pages 293-296. 
