„,,,^^.pT GEOLOGY NATURAL DIVISIONS. 85 



of Golden City, is also cine mucli information about the coal-openings 

 of the Territory. 



Eapid preparation of a summer's observations, together with absence 

 during publication, must atone for many errors which will inevitably 

 occur. 



Yery respectfully, yours, 



AEOH. E. MAEYINE, 

 Assistant Geologist^ Directing the Middle Parle Division. 



Dr. F. Y. IlAYDEN, 



In CMrge of the United States Geological 



and Geographical Survey of the Territories. 



CHAP TEE I. 



AEEA, NATUHAL DIVISIONS, AND PRINCIPAL TOPOGRAPHICAL PEATUEES. 



The territory embraced by the survey of the Northern or Middle Park 

 division during the summer of 1873 is included between the parallels of 

 39° 30' and 40° 20' north latitude, and the meridians of 104o 45' and 106o 

 30' west longitude. It forms, therefore, a rectangular area, which has 

 a width north and south of about sixty miles, a length east and west of 

 about ninety-three miles, and which contains nearly five thousand six 

 hundred square miles. This area may be conveniently located by refer- 

 ence to the Union Pacific Eailroad, which passes about fifty miles north 

 of its nprthern border, on which side it is directly joined by a similar 

 area of the fortieth-parallel survey. 



The eastern portion of this parallelogram rests upon the western border 

 of the great plains which extend uninterruptedly from the Mississippi 

 and Missouri Eivers, and includes some of the most thickly settled por- 

 tions of Colorado. Denver City, and many smaller towns, with their 

 railway connections and surrounding agricultural regions, are here sit- 

 uated. 



The middle portion of the district is traversed north and south by one 

 of the highest portions of the main continental divide, with, on either 

 hand of the great crest, the accompanying zone of mountain country, 

 which contains nearly all of the more important metal-mines of the Ter- 

 ritory. 



The western portion of the area is chiefly occupied by the several 

 depressions which together make up the Middle Park, with their sepa- 

 rating ridges, and is limited upon the extreme west by the Park range. 

 A strip along the southern edge of this portion was not completed by 

 this season's work. Eegarding its broadest topographical features, 

 therefore, this rectangle may be considered as dividing itself naturally 

 into three i)ortions, which we may conveniently designate as the eastern 

 portion, or the plains, the middle portion, or the mountains, and the 

 western portion, or the park. So far as drainage alone goes, the dis- 

 trict as a whole may be regarded as being separated by the nearly north 

 and south crest of the main mountain-divide into two very nearly equal 

 east and west portions, in each of which the drainage system is quite 

 simple. The main artery upon the east is the South Platte Eiver, which 

 flows diagonally northward and eastward across the i^lains portion of the 

 district. All the streams rising at the eastern base of the main central 



