123 TRANSACTIONS OF THE [.TAN. 18, 



River, to the south tit Aliniosa. In their structure, they do 

 not resemble the connected ranges. 



We enter this group of mountains by railroad at Durango, 

 coming in over the northern rim of an extensive cretaceous coal 

 basin. We see coal out-croi)})ing high uji in the hills. 



Going up the Aninuis river, cliffs and hills of stratified sand- 

 stone rocks are seen on either side. They all dip southward, and 

 are of various colors; white, gray, and red are predominant. At 

 Tremble Springs, we find in the cliffs back of the hotel, a black 

 limestone filled with Afhyris subtilita. Here we also found two 

 species of Productns. Tliese fossils are sufficient to identify the 

 age of the rocks as Carboniferous. These rocks extend up the 

 Animas to Kockwood, where they rest on Silurian, and this on 

 red granite. The canon of the Animas, with its ])erpendicular 

 walls, is in granite. Metamorphic rocks come in near the first 

 railroad bridge and reach toNecdleton. Tlie Needle mountains 

 are quartzite. In Pidgeon Peak and Mt. ^-Eolus, they rise in 

 shar[) peaks of quartzite 12,800 feet above sea level. 



From Needleton on up the Animas caflon, we are in metamor- 

 phic rocks. We emerge into the beautiful alpine valley of Sil- 

 verton, formerly Baker's Park, through a gap between Sultan 

 Mt. on the west, and Mt. Kendall on the east. On the latter, 

 near the railroad bridge, we can see a large patch of white dolo- 

 mite, which was caught in the grip of elevating forces, and car- 

 ried up with it. The main mass of this mountain on the south 

 is metamorphic. 



Sultan Mt. is an altered sandstone, with slight dip northward. 

 It is full of dykes and cliffs of porphyry, and capi)ed with tra- 

 chyte. The porphyry is colored blue or ]nirple, and composed 

 of conglomerate and brecciated rock. Topograjihically, this 

 is the centre of the San Juan Mts. Here we first see whole moun- 

 tains injected with molten rock, and charged with veins of sil- 

 ver-bearing ores. The natural structure of the mountains is de- 

 stroyed, and it is only at rare intervals that we can find any 

 remnant of their original condition. Mola's mine is on the 

 .south slope of Sultan. The Williams' grouji is at the base of 

 the east sloi)e, while the North Star extends from near the sum- 

 mit to the base. 



In the rear of Sultan, on a branch of Mineral Creek, is 

 situated Bear Mt. Its bright red strata of saiulstone lie nearly 

 horizontal, and it is capped with sharp spires of gray trachyte. 

 This mountain gives us a clue by which to interpret all the other 

 mountains of the group. From Silverton to Ouray, by route of 

 Mineral Creek, is twenty miles. Tiianks to Mr. Otto Mears, a 

 graded turnpike road has been built along the valley of Mineral 

 i-!reek, giving a fresh cut of the rocks the entire distance, and 



