1758 REPORT OF THE CHIEF OF ENGINEERS 



the farthest north of the mineral ranges of the San Juan, and has furnished some of 

 the finest mines of the entire region. 



The Uncompahgre Peak is, in its eastern part, the highest point, rising to beyond 

 14,400 feet, a huge precipitous mass. In its western portion, Mount Sneffles is the 

 loftiest, being over 14,200 feet. The latter is the best known and the most important 

 in the range on account of its mineral wealth. This has been mapped as Blaine's 

 Peak, in honor probably of the distinguished Senator, in some of the maps of this 

 region. By the latter name, however, can it scarcely be perpetuated, as it has long 

 been differently called, and, being the seat of most valuable and important mining 

 deposits, by official records it has been otherwise designated. 



To the northwest the mountains slope into the Great Uncompahgre Plateau, the 

 divide between the waters of the San Miguel on the west and those of the Uncom- 

 pahgre and the Gunnison on the east. The slopes on the mountains on the immedi- 

 ate north are excessively steep, and the streams have cut most frightful canons. Not 

 content with penetrating the deep volcanic rock, they have passed into the underlying 

 sandstones. 



The lovely park of the Uncompahgre lies below, inaccessible, intervening a succes- 

 sion of precipices, dashing torrents, huge bowlders, and fallen timber crossed and 

 interwoven. The only passage-way is a steep and dangerous trail leading through 

 this scene of desolation, and the journey to the park below is not made without per- 

 sonal danger. 



Beyond stretches the great and rich agricultural valley of the Uncompahgre, from 

 which settlers are now interdicted, it being upon the Indian reserve. 



THE RIO GRANDE SLOPES. 



Between masses of trachyte, looking down upon the river, until we have reached 

 Wagon Wheel Gap, the Rio Grande is descended. 



The most remarkable geological formation, en route, attracting the attention of the 

 passer-by, is the Bristol Head, the southern and culminating point of the Bristol 

 Plateau, a great trachytic mass stretching from the continental divide, about 10 miles 

 to the south, to the vicinity of the Rio Grande, perhaps 2 and 3 in width, aggregating 

 almost 30 square miles in area, with an altitude of nearly 13,000 feet. Along its west- 

 ern base, in the canon, passes the highway from the Rio Grande to Lake City, about 

 3,000 feet beneath its summit, the sides of the cliff being mainly vertical. The upper 

 part of the < of the great divide, rising to 13,000 feet, is known under the name of 

 the Cochetopa Hills, from the stream flowing down the western slopes, an affluent of 

 the Gunnison ; its eastern sides are drained by Saguache Creek flowing east and south- 

 east into San Luis Creek and the San Luis Lakes. The divide between this and the 

 waters of the Rio Grande is called La Garita Hills. These sections exhibit in strong 

 degree the plateau formations, which, cut by the various streams, give unusually steep 

 though short canons and abrupt mesas. An extension down to and closing upon the 

 Rio Grande is reft by the river, giving that attractive little canon and interesting 

 point called Wagon Wheel Gap. 



Section III. — Trends of ranges and slopes. 



The direction of the surface inclination is an important function of the value of 

 certain sections for agricultural and other purposes. The trend of the Sangre de 

 Cristo Range to the northwest gives its surfaces within the valley more of southern 

 warmth than would otherwise obtain. Snow is found often upon its eastern sides long- 

 after it has disappeared from that at the same altitude upon the valley slopes. This 

 is more noticeable when directly considered with reference to the San Juan Mountains. 

 Contrasting the valleys of streams upon the northern flank, affluents of the Rio 

 Grande, with those upon the southern trend at the same altitude, we find the valleys 

 of portions of the San Juan, Weeminuche, Los Pinos, Vallecito, the plateau of the 

 Upper Piedra, &c, notable examples which have been personally observed in regard 

 to greater luxuriance of vegetable growth. Its effect extending slightly to climatic 

 influences gives those upon the northern inclination a more lengthened wintry aspect 

 than the others. We have frequently come upon hillocks in a contiguous region of 

 an altitude approaching 8,000 feet, where in January the northern slope would be cov- 

 ered with nearly a foot of snow, while upon the southern the surface would be quite 

 bare, affording fine grazing for the deer and elk. Traveling in higher regions during 

 a series of snow-storms of continuance, this was more strongly substantiated. 



Considering this influence extended to the regions above, we find the snow falls 

 earlier, lies longer upon the ground, and there is less area adapted to the growth of 

 cereals than in that upon the southern flank of the mountains Avith otherwise similar 

 situations. 



It has been stated that for a rise of 300 feet there is a decrease of temperature of 1 

 degree. This rule is not, however, arbitrary ; there are many causes operating as 



