1912] WATSON— PLANT GEOGRAPHY OF NEW MEXICO 195 



posed of sand, hard adobe, or a clayey gravel with stones up to 

 the size of a man's head thickly strewn over the surface ; or, more 

 usually, all of these deposited in alternate layers, showing plainly 

 its fluviatile origin. On the west side of the valley are occasional 

 sand dunes bearing absolutely no vegetation. . 



From these hills a clinoplain, known locally as "the mesa" 

 (not a true mesa), slopes gradually upward toward the mountains 

 with a quite uniform grade of nearly ico ft. to the mile, although 

 appearing to the eye to be nearly level or gently undulating. This 

 mesa is also of stream origin, consisting of the ancient gravels 

 and clays of the Rio Grande intermixed with sand fans and other 

 detritus resulting from the weathering of the mountains. On the 

 east this plain stretches for nine or ten miles to the base of the 

 Sandia Mountains, forming one of those old western river valleys 

 so admirably described by MacDougal. 2 Every two or three 

 miles this mesa is crossed by a sandy "arroyo," or dry stream 

 bed, which once or twice each summer becomes a raging torrent 

 for an hour or two. These arroyos lie in shallow valleys, the 

 largest, however, having banks 100 ft. or more high. Few of these 

 arroyos reach the river proper, but spread their flood waters over 

 the floor of the recent valley, building up fans of an alluvium- 

 like clay at their mouths. Numerous smaller arroyos head on the 

 "mesa" proper or on its dissected edge. A similar mesa on the 

 west side of the valley is partly covered by a flow of lava so recent 

 that it has suffered almost no weathering, the shallow soil that 

 covers it, to the depth of a few inches, having been deposited by 

 the wind. A mile or so back from its edge this lava field is sur- 

 mounted by five volcanic cones, the largest being about 300 ft. 

 high. 



From the eastern mesa the Sandia and Manzano mountains 

 rise rather abruptly, sand and gravel fans at the mouths of the 

 canons forming a transition. The range is a typical block mountain 

 with the principal fault at its western edge. It is composed of 

 Archean granites and schists, capped by a layer of Carboniferous 

 limestone 50-200 ft. in thickness. This limestone shows a dip to 

 the southeast of about 20 . To the east of the main ridge lie 



2 MacDougal, D. T., Botanical features of North American deserts. 1908. 



