TO -JUNCTION OF GRAND AND GREEN RIVERS. 39 



eastern base oi the Valles, the mountain-chain forming the western boundary of this 

 portion of the valley of the Rio Grande. In this lava-plain the Rio Grande has 

 excavated the canon through which it flows above Santo Domingo, and its cut edge 

 forms the high mesa-wall which borders Galisteo Creek on the north, from the Cerril- 

 los to its month. 



It is only near the. Rio Grande that the Tertiary beds to which I have referred 



make their appearance. Here they are altogether similar to those I have described as 

 occurring on the Arkansas and other localities east of the mountains, viz, white tufa- 

 ceons limestones and beds of conglomerate, the pebbles mainly of pumice or trap, and 



all without fossils. 



On both sides of Galisteo Creek the Cretaceous strata contain beds of lignite. 

 On the smith side, in the Ibot-liills of the Placer Mountains, an outburst and overflow 

 oi trap has, over a limited area, changed the lignite into bright, hard anthracite 

 coal. This will doubtless have considerable economical value. The bod is four to 



five feel thick, included between strata of bluish argillaceous shale, with bands of 



nodular iron-ore, the whole forming an accurate copy of the most characteristic sec- 

 tions afforded by the Coal-Measures of the .Mississippi Yallov. 



The Placer .Mountains form a grout) rather than a chain, their extent east and 

 west being nearly or quite equal to that north and south. They are made up, how- 

 ever, of a series oi short lines of upheaval, having a trend somewhat north of west 

 and south of east. Like most isolated mountains and mountain g roups, as distinguished 

 from the chains which have come under my observation in Central and Western North 

 America, they are composed, for the most part, of rocks decidedly eruptive in charac- 

 ter, and to a much less degree than their neighbors the Sandia and Santa rY Mount- 

 ains, consist of granite Or stratified materials. 



The principal constituents of the Placer Mountains arc; porphyry, trachyte, tufa, 

 and trap, with masses of granite and mica-slate. About their bases, and in some cases 

 upon their flanks, the stratified rocks of the country may all be found, but are less 

 fully exposed than in the more important mountain ranges of this region. The Placer 

 Mountains have received their name from the gold which they furnish; considerable 

 quantities having been taken during a long period of years from the placers of their 

 bases and slopes. More recently, systematic mining operations have been commenced 

 in the auriferous rocks, and a limited amount of gold has been extracted from them. 

 The gold of the Placer Mountains is of remarkable purity, and there is no question 

 but that a ureal deal of it exists there, yet it is quite doubtful whether these gold-dig- 

 gings will ever become very productive, from the fact that it will be difficult, if not 

 impossible, ever to bring. to the placers an adequate supply of water, and the gold is 

 so sparsely and widely distributed through the metalliferous rock that mining and crush- 

 ing it will be but moderately compensated by the quantity obtained. 



The "Old Placer" has been worked by the Mexican population since the first occu- 

 pation of the country by the Spaniards, and probably was a place of resort by their 

 predecessors the Pueblo Indians. The gold is obtained in the bed of an intermittent 

 stream flowing through a ravine which separates two of the principal peaks, and in and 

 beneath the debris which has accumulated where this gorge opens into the valley of 



