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that react readily with ground water. Ground water flowing through shale, thus, becomes highly mineralized. in the central part of the basin, geologic formations consist of subequal amounts of sandstone and shale, and ground water there is intermediate in composition and quality between ground water on the western and eastern sides of the basin. Regional trends in ground-water quality are disrupted by topo- • graphic and geologic features. As ground water flows from stream divides to stream valleys, it gains dissolved solids (fig. 8.1-2). Human activities, such as mining or irrigated agriculture, also add dissolved solids to ground water in valleys. Ground water in valleys with intermittent or ephemeral streams, is thus, more mineralized than in stream divides. In valleys with perennial streams, the stream flow may be sufficient to flush out dissolved minerals and cause ground water in alluvium and shallow bedrock to be better in quality than in adjacent bedrock. Ground water in volcanic-capped mesas is low in dissolved solids because the volcanic rocks aiao Gre. Ver~r ,$3'Mea~ unJ mpidly+runsm~t+he. /If}a~a~-q~rauwd water through them. The origin of highly mineralized sodium chloride ground water near the Spanish Peaks, an intrusive [tvnnocifren cwt stock, and related dikes is obscure, but the,~ground water may result from reactions with the igneous rocks. Geologic formations are distinguishable by their ground-water quality. The Devils Hole-Fa=asita, Poison Canyon-Cuchara and volcanic rock aquifers contain mostly calcium bicarbonate water with less than 500 milligrams per liter of dissolved solids. The Huerfano Formation contains mostly calcium sulfate water with 500-1500 milligrams per liter of dissolved solids. The Trinidad-Vermejo-Raton aquifer contains mostly sodium-bicarbonate water with 500-1500 milligrams per liter of dissolved solids. Cretaceous shale, limestone, and sandstone contain 67