28 BUREAU OF AMERICAN ETHNOLOGY ibull. 54 



large area. This material was doubtless ejected from a volcanic 

 vent or vents, whether as mud or as ''ashes" may be questionable. 

 At the Rito de los Frijoles the deposit is 500 to 600 feet or more in 

 depth. The exact conditions under which it was deposited are not 

 easy to determine. In fact, in the great area covered by the tufa 

 conditions would be almost certain to differ in different localities. 

 Furthermore, it is not at all likely that the material was all ejected 

 during a single eruption, and some of it may have been ejected in 

 one condition and other portions in a quite different condition. 

 Wherever the writer had opportunity to examine it, the upper part 

 was very much like the lower part in constitution, but differed uni- 

 formly in being of somewhat greater specific gravity, hardness, and 

 color, a matter which is discussed farther on. The high vertical cliff 

 at the Rito gives one the impression that perhaps the whole softer, 

 more homogeneous portion below may have been the result of one 

 period of deposition and that the upper half or so of the formation, 

 with its alternating hard and soft zones, may represent another 

 period, yet the absence of any unconformity in material so easily 

 eroded makes it difficult to believe this to be the case, unless the 

 interdeposition period were very short indeed, or the slopes were so 

 gentle as to permit only tliQ^ most sluggish movement of surface 

 waters. 



Popular opinion in the region regards the formation as volcanic 

 ash or volcanic mud deposited in a large body of water. Speaking 

 of portions of the basin not examined by the author, some scientific 

 wi'iters have expressed the same opinion. At the Rito de los Frijoles 

 the character of the material does not seem consistent with that view. 

 It would be rash to express any definite opinion applicable to the 

 whole region without examining the area in detail. The ground mass 

 at the Rito is light, but variable in specific gravity, some fragments 

 almost floating when placed in water. Especially is this true of the 

 numerous pumice fragments, some of them several inches in diam- 

 eter. These are scattered in great numbers throughout the forma- 

 tion from base to- top, mingled with coarse, angular fragments of 

 heavy rock, including obsidian. It does not seem at all likely that 

 such material, differing so greatly in specific gravity and size, could 

 have been deposited in a large body of water without a considerable 

 degree of assortment, yet a search along the canyon walls at the Rito 

 failed to disclose any defuiite evidence of the assortive action of 

 water. At one place in Alamo canyon a small local deposit of stream- 

 assorted tufa pebbles was found, just such as would be expected in 

 places if deposition had been subaerial, whether by successive showers 

 of volcanic ashes or flows of volcanic mud. It does not seem prob- 

 able that the formation is the result of a single eruption. Between 

 eruptions some erosion and redeposition of material would occur, 



