iiKNDEKsoN] GEOLOGY AND TOPOGRAPHY 29 



even though the inter-eriiptioii periods were of short (hiration, for the 

 waters of the Rio Grande and other streams must have continued to 

 flow somewhere in the basin affected by the tufa. The banding of 

 the upper part of the tufa is not easy to understand. If the bands 

 signifv'' successive tleposits, there should be marked unevenness in 

 evidence of local erosion between the bands, which the writer has 

 failed to find. If the deposition occuiTetl in a large body of water of 

 sufficient depth to avoid wave erosion, that fact might account for this 

 point. The extent of tiiis banding, evidenced by the bluffs and ter- 

 races hereinafter discussed, makes it evident that any such body of 

 water must have been of great length, .30 or 40 miles in width and at 

 least 1,000 feet in depth. If any barrier has been found which would 

 account for such a body of water the writer has not learned of it. 

 This and the lack of assortment of the material constitute very 

 strong, though possibly not conclusive, evidence against the idea 

 of deposition in water. On the whole, the impression Tlerived from 

 an examination of the tufa is that it consists of a series of flows of 

 rather thin mud, inclosing the fragments of pumice, obsidian, and 

 other rocks, though before this suggestion is accepted as final much 

 more work should be done. 



The tufa contains innumerable quartz crystals one to three milli- 

 meters in diameter. In weathering, the soft, fine material in many 

 places is washed from the quartz crystals, leaving small deposits of 

 clear crystalline quartz sand in the beds of gullied. Many ant hills 

 two or three feet in diameter are composed almost entirely of the 

 crj^stals; they are often seen glistening brightly in the sunlight. A 

 careful examination of the weathered-out crystals in most cases 

 shows that the facets are clear, bright, and sharp, but one side is 

 usually broken away. It is quite probable that this is due to diurnal 

 expansion and contraction, as the heat of the sun at midday is 

 intense, while the nights are cool. The diurnal changes in tempera- 

 ture are not so eft'ective in the disintegration of the tufa as in case of 

 some other rocks, because it is somewhat elastic, so that the scaling 

 ofT so noticeable in some of the granites and sandstones under such 

 circumstances is not noticed here. 



The quartz crystals fail to explain the origin of the tufa. It is 

 believed that they were formed in the magma b}^ its partial crystal- 

 lization before it was ejected, and they would occur in their present 

 condition probably whether the tufa were ejected in the form of ashes 

 or of mud. The same sort of material may form volcanic ash or 

 volcanic mud, according to the conditions under which its expulsion 

 occurs. If violently ejected in a somewhat dry condition by an 

 explosion it forms what is called volcanic ash, but if exuded in a 

 thoroughly wet condition it comes out; of course, as mud. 



