TABLE OF CONTENTS. y 



Page. 

 Keport of F. M. Endlich.— Continued. 



On the eruptive rocks of Colorado — Continued. 

 Chapter III. Volcanic eruptlves — Continued. 



Rhyolite , 229 



Porphyrit: c trachyte ][ 232 



R^sum^ of Pori^hyritic trachyte. 240 



I. Location 240 



IL Physical appearance 04^ 



III. Structure 241 



IV. Relations to sedimentary formations 242 



V. Influence upon sedimentary strata 243 



YI. Correlation with other eruptlves 244 



vn. Age ;;; 245^ 



Basic volcanic eruptlves 245 



Dolerite 245 



Basalt 246 



I. Massive 246 



II. Anarhactic 250 



III. Isolated 250 



Ch apter IV. Age, com parison , and origin of eruptlves , 252 



Age of the eruptlves of Colorado 252 



Comparison of eruptlves of Colorado with those of other regions 255 



' Origin of erupted rocks „ 262 



Origin of the erupted rocks of Colorado 269 



Recapitulation 272^ 



PART II.— TOPOGRAPHY. 



Report of A. D. Wilson, Chief Topographer : 



Letter of transmittal 275 



Report on the primary triangulation of Colorado: 



Chapter I. Primary triangulation 277 



Measurement of the Denver base 2.'!0 



The San Luis base 281 



Azimuth of San Luis base 282 



Method of adj usting the triangulation 282 



List of primary triangulation stations, &c 285 



Tables 286 



Chapter II. Methods of topographical field and office work 298 



Determinations of altitudes 299 



Methods used in determining the elevation of points 299 



Office-work of the topographer 303 



Declination of the magnetic needle 306- 



Report of Henry Gannett, M. E. : 



Letter of transmittal 311 



Report on the arable and pasture lands of Colorado : 



Chapter I. Physical geography 313 



Topography 313 



Climate 315 



Chapter II. Estimated distribution of arable and other lands in Colorado. 318 



General considerations 319 



Amount of water used in irrigation 320 



Proper season for gauging streams 321 



Chapter III. Cultivable areas of Colorado 323 



The North Platte drainage area 323 



Drainage area of the South Platte 323 



Drainage area of the Arkansas 326 



The San Luis Valley 328 



Drainage area of the San Juan River 330 



Drainage area of the Grand River 332 



White River , 337 



Yampa River 338 



Chapter IV. Irrigation in Colorado 339 



Irrigation-ditches 339 



Cost of irrigation 341 



Reservoirs 341 



Chapter V. Grazing-lands of Colorado .... 344 



Principal grazing areas --• 344 



