76 SURVEY or COLORADO AND NEW MEXICO. 



ern portion of tlie valley is called the San Luis Park proper. This 

 northern portion, above the bow of the Rio Grande, is about sixty miles 

 in length, and has an average width of fifteen to twenty miles. About 

 the center of this park is a singular depression, about ten miles wide 

 and thirty miles long ; it looks like oue vast thicket of " grease wood," 

 Sarcohatusvermicularis, and other chenopiaceons shrnbs. Into itflow some 

 twelve or fifteen good sized streams, and yet there is no known outlet, 

 neither is there any large body of water visible. It seems to be one vast 

 swamp or bog, with a few small lakes, one of which is said to be three 

 miles in length. Although entirely disconnected from any other water 

 system the little streams are full of trout. 



On the south side of the Sierra Blanca the foot-hills are composed of 

 the light-colored marls, and on the west side of the mountain, and near 

 Mosca Pass, are the sand hills, which are composed of the loose materials 

 of this formation. 



Here also is another couspicuous remnant of it left after erosion. On 

 the west side, just below Sa watch Creek, and in the Kincon, are 

 some rather high hills of this marl at the base of the mountains. The 

 materials thrown out of the excavations of prairie dogs show that the 

 valley is entirely underlaid with it. I am convinced therefore that this 

 fresh-water deposit occupied the whole of this valley from Poncho 

 Pass to the mouth of Gallisteo Creek, and how much further southward 

 I cannot tell, but there is evidence that it extends, either continuously 

 or with interruptions, through Xew Mexico, and even further. 



From Fort Garland to the Poncho Pass no sedimentary rocks of older 

 date than the marls are seen along- the margins of the mountains on 

 either side until we reach Kerber's ranclie, al out ten miles below the 

 summit of the pass. On the west side of the valley, on the foot-hills, 

 is a large thickness of carboniferous limestones, lifted high on the summits, 

 and dipping east at an angle of fifty degrees. This limestone continues 

 only a few miles, and is another of the remnants that are left of the sedi- 

 mentary rocks among the mountains. 



Commencing at Fort Garland, the range of mountains that wall in the 

 San Luis Park on the east side is grand in its i)roi)ortions. From the 

 Sierra Blanca nearly to the Poncho Pass it appears to be purely eruptive, 

 and to be comiiosed of a series of ranges or axes trending nearly northeast 

 and southwest. At the northern end the eruptive portion ceases, and 

 the lower metamorphic moimtains flex around so as to trend northwest 

 and southeast. On the west side, the mountains are far less lofty, but 

 they seem to form a nucleus of metamorphic rocks, with avast number of 

 dikes, from which the basalt has i^oured over nearly the entire region. 

 All the foot-hills south of the Sawatch are composed of erni^tive rocks, 

 but north of that point the gneissic rocks are seen. This range of moun- 

 tains seems to be made np of a number of smaller ranges, with a general 

 trend ncJrthwesf and southeast. It would seem that where a range of 

 mountains is purely eruptive the minor ranges trend northeast and south- 

 west, but that where there is a metamorphic nucleus the eruptive mate- 

 rials follow the strike of the minor ranges. 



At the summit of the pass the hills are grass-covered and the road 

 excellent, but the nucleus of the mountains on the east side is meta- 

 morphic, with dikes of eruptive rocks everywhere. The little stream, the 

 valley of which we descend, flows through a monoclinal rift or interval 

 between the ridges of metamorphic rocks. 



About two miles from the summit this little branch is joined by the 

 main fork, and the whole continues to flow through a monoclinal valley 

 until it empties into the South Arkansas. The main Poncho Creek rises 



