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INTRODUCTORY REMARKS. 
THE general map accompanying the reports of explorations for railroad routes from the 
Mississippi river to the Pacific ocean embraces all the territory of the United States from the 
great lakes and Mississippi river on the east to the Pacific ocean on the west. The plan 
adopted in constructing it has been to represent only such portions as have been actually 
explored, and of which our information may be considered reliable. It has been, frequently 
necessary to decide between the merits of discrepant authorities, and to select those which 
seem the best. In performing this delicate task, the general principle has been carried out of 
adopting the work of those explorers who were best provided with instruments, and who 
possessed the largest share of that experience which is so necessary in attaining accuracy, 
taking the evidence of these advantages from their own reports. 
The compilation has been made from the Pacific railroad explorations and surveys, and 
from all other reliable maps and works that were available. The determinations and 
surveys of the United States Land Office, United States Coast Survey, and United States 
Mexican Boundary Commission have been obtained in advance of their publication, and, in 
return, copies of this map have been furnished whenever requested, at all stages of its 
progress. Many authentic manuscript maps have been received from the Bureau of Topo- 
graphical Engineers, where the systematic arrangement and cataloguing renders them easy for 
reference. Others have been furnished by the offices of the Adjutant General and Quarter- 
master General of the army. Ihave also been favored with maps and books from the Indian 
Bureau, from the Smithsonian Institution, and from the library of Colonel Peter Force, of this 
city; and with notes, letters, sketches, and suggestions from officers of the army and others. 
Many of the maps and reports used have never been printed, while numbers are now out of 
print and difficult to obtain. There is no library or office of the government in which a 
complete series of these works can be found. 
Before detailing the manner in which the compilation has been made, I have therefore 
thought it desirable to give a brief account of each of the different explorations, the routes 
traversed, the methods employed in observing, the maps prepared, &c., &c., in-order of date. 
By this undertaking I hope to promote the consultation of the original reports and maps, by 
pointing out to each investigator those works which probably contain information about the 
region of country especially interesting to himself. As a general rule, I shall confine myself to 
the explorations made in the territory of the United States. 
The maps of the old Spanish and French navigators and explorers who visited the Mississippi. 
the Gulf of Mexico, and the shores of the Pacific, and who often examined portions of the 
interior, have nearly all been replaced within our territory by more accurate determinations of 
our own. They have, therefore, little practical value in this connexion, and will not be 
specially noticed. An almost complete account of Spanish discoveries in New Mexico prior to 
1811 can be found in Baron Humboldt's New Spain. The subject is still further discussed in ` 
