202 BOTANICAL GAZETTE [September 



A little higher up, where the arroyo is not over 6-8 ft. wide, 

 the bed proper is generally free from plants except an occasional 

 Euphorbia, but the banks are occupied by Bigelovia. Near its 

 head, where the arroyo is only 1-2 ft. wide, its sides are occupied 

 by a narrow fringe of shrubs, chief of which are Parosela formosa 

 and Lycium pallidum. 



In the valleys of the larger arroyos that continue the mountain 

 streams there appears yet another society, characterized by the 

 dominance of either Suaeda Moquinii, or the greasewood Sarco- 

 batus vermiculatus, or both, the former being more particularly 

 confined to the adobe fans at the mouths of the arroyos. Like 

 so many of the shrubby plants of this region, these and especially 

 the Suaeda catch the wind-blown dust and allow it to accumulate 

 among its stems, making mounds like low sand dunes, but in this 

 case composed of adobe. For this reason this association is covered 

 with hummocks often 6 and sometimes 10 ft. high. This is an 

 alkali society, due to the evaporation of the flood waters of the 

 arroyo, and has the same relation to the arroyos as a floodplain 

 forest to a river valley in the east. Mixed w r ith salt grass it is the 

 dominant association around the salt beds and lakes of the Estancia 

 Valley, as well as along the Rio Salada branch of the Jemez River. 



MESA FORMATION 



the more level ground of the mesa 



stops abruptly at its dissected edge, as stated under the last head- 



ing. This 



where it has not been too seriously over-grazed. It should prob- 

 ably be classified as a steppe. Now, thanks to lack of scientific 

 control of grazing, it has been so invaded by the composite Gutter- 

 rezia (a somewhat shrubby perennial that grows to be 3 ft. high, 

 and is often called "goldenrod") as to merit being called a Gutter- 

 rezia formation (fig. 1). The seasons of 1909 and 1910 were drier 



July 



most of the remainder 



small 



utumn 



mesa 



those two vears it bloomed onlv in the mountains 



