GEOLOGICAL PAPERS. 145 



wealth and for permanent civilized, prosperous and increasing popu- 

 lation. They have restricted the fornfationof wide alluvial valleys and 

 covered large areas of possible agricultural surfaces, converting them 

 into sterile deserts. By damming the Rio Grande in late Tertiary 

 times, it made the valley of that stream a great longitudinal lake or a 

 series of linear lakes for several hundred miles of its course, which 

 became filled with the detritus of basaltic and granitic masses to the 

 depth of nearly 1000 feet. Thick layers of basalt run into its bed 

 and spread on both sides of it, the stream finally cutting deep can- 

 yons to deliver itself. Much of the drift and sediments yet remain, 

 as well as most of the broad, barren lava tables. The remains of the 

 old lake deposit are seen to be stratified and sometimes feebly consoli- 

 dated with calcium carbonate. The layers consist of silicious sand 

 alternating with fine to very coarse conglomerate. The water- worn 

 pebbles of the latter are often six to eight inches in diameter. Every 

 description of color and composition of plutonics is found in the 

 smooth stones that cover the mammillary faces of the low hills border- 

 ing the lower levels of the valley. One may sometimes find gold, 

 which the naked eye can detect in the specimens he gathers, and 

 many are colored by ores of copper or other metals. The cutting of 

 the Rio Grande canyons resulted in several debacles, according to the 

 evidence of distinct benches along the boundaries of the lake basin, 

 especially marked in Colorado, but also manifest in New Mexico. 



The granite of the mountains is mainly a feldspar porphyry, with 

 the feldspar ingredient in rather large crystals. In the Sangre de 

 Cristo this component is reddish, also in the Nacimiento and some 

 other ranges, but further south it is oftener a white orthoclase. The 

 granite passes into gneiss or vice versa in places, and where schists 

 are in contact it is often that the junction is not a locus that can be 

 identified. Quartz and granite dikes are frequently found cutting 

 into the great axial masses. About fifty-five miles straight north of 

 Santa Fe, near the Denver & Rio Grande railroad, is a granite dike 

 carrying reddish feldspar in masses of four to five cubic feet, quartz 

 masses as large or larger, and white mica plates several inches thick. 

 The probability is that this dike was not thrust up to the surface, but 

 cooled very slowly under great pressure, and is exposed through denu- 

 dation by erosion of thick strata. 



The gneisses, schists and granites show more or less lamination in 

 the vertical direction, and some schists under the lens exhibit clastic 

 structures. Apparently they do not conform with the layers of con- 

 glomerate and quartzites that rest upon them on the sloping flanks 

 of the ranges, when viewed from the escarpment faces. They are be- 

 low the Carboniferous and are supposed to be Algonkian metamor- 

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