GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 231 



The third district, which is the San Luis Park, belongs both to New 

 Mexico and Colorado, and cannot be divided into parts corresponding 

 with the arbitrary line of division betv/een these two Territories. 



The fourth division I shall make is not a separate district, as each of 

 the others, but includes the other parks and the small amount of arable 

 land in the mountain valleys, which, on account of the proximity of some 

 of them to the mining districts, become imi}ortant, notwithstanding their 

 small extent. This may be called the mountain district. 



It will be seen that each of these three natural districts has its great 

 river by which it is drained ; the Denver district finding an outlet foi 

 its waters through the South Platte ; the Arkansas district through the 

 Arkansas River 5 the San Luis Park through the Eio Grande. And as 

 we descend to the examination of the more minute divisions of these 

 larger districts, we must follow the natural arrangement of streams and 

 valleys. 



THE DENVER DISTRICT. 



This district is naturally divided into two sections; the first includ- 

 ing the territory north of the South Platte and between it and tlfe moun- 

 tains; the second, the territory between the Platte and the Divide. 



As the first section presents more definitely and sharply the peculiar 

 features of this country which bear upon its agriculture than any other 

 portion, I will give a somewhat minute description of it. 



The Platte, leaving the mountains some twenty miles southwest of 

 Denver, after bearing out a short distance on the plains, runs northeast, 

 slightl^^ diverging from a parallel course with the east range of mountain#, 

 for a distance of about eighty miles, where it is joined by the Cache a la 

 Poudre, and then turns eastward ; thus giving the section a triangular 

 shape, with, the north side of the Cache a la Poudre valley as its base, 

 the mountains for one side, and the Platte the other. Its general sur- 

 face is a broad i^lain sloping from the mountain flank eastward to the 

 river level with valley furrows, and rounded, low ridges traversing it 

 from west to east. 



The various streams which take their rise in the mountains east of 

 the great Divide, between the waters of the Atlantic and Pacific, run 

 nearly an eastern course until thej' unite with the Platte. 



The first debris, and all the heavier materials, brought down from the 

 mountains since their upheaval, have, as a matter of course, been depos- 

 ited near the base. Hence as we recede from the mountains towai'd the 

 east, this local drift decreases in the size of its particles and depth of 

 deposit. Over this is deposited the alluvial stratum forming the soil, 

 which, close to the base of the mountain, but thinly covers the 

 boulder drift, but increases in thickness eastward. The creeks rush- 

 ing down more rapidly near the mountains, cut deeper furrows through 

 this deposit near the base than at a distance from it. In consequence 

 of this, the terraces or ridges, which lie between the streams, are highest 

 above the water near the mountains, decreasing as they recede from it ; 

 that is, the distance between the water level of a stream and the top of 

 the terrace which flanks its valley is more, half a mile from the foot 

 of the mountain than it is ten miles from the foot. This fact in other 

 parts of our country might have very little bearing upon agriculture, 

 bat it is a consideration of vital importance to the Colorado farmer, who 

 must irrigate his land or receive but little return for his labor ; tor when- 

 ever this is the case it is evident that at some point, the water can be 

 cari'ied to the top of the bordering terrace. 



