12 REPORT UNITED STATES GEOLOGICAL SURVEY. 



surface is approximate!}^ level ; the under surface takes the form of the 

 depression into which it flowed, which, as shown by the section in the 

 cliii" face, had a gentle slope from the northern rim to the south. The 

 sandstones dip in the opposite direction, that is, to the north, at an an 

 gle of about 15<^. The ends of these strata had been planed down to a 

 l)retty uniform slope before the flow of the rhyolite occurred. 



The most peculiar and interesting circumstance connected with this 

 flow is the existence of a bed of loosely compacted material that lies 

 between the sandstone floor and the sheet of rhyolite. This bed varies 

 in thickness from 5 to 10 feet. The lower part is deposited upon the 

 somewhat uneven surface of the sandstones, and is composed of a white 

 powder that resembles slacked lime in appearance, and is probably a 

 volcanic tufa. The white stratum runs up into a delicate gray stratum, 

 which is succeeded by about 4 feet of fine grained laminated tufac^eous 

 sandstones that have the most perfect lines of bedding possible. There 

 is an alternation of white, gray, and dark gray lines shown uj)on the 

 smooth vertical surface that for perfection of line and delicacy of 

 color could not be rivaled by the pencil of the skilled artist. That these 

 lines indicate planes of bedding is clear, as there is a slight change in 

 the coarseness of the material noticeable in the various layers. The 

 most remarkable fact seems to me to be this, that these lines of color, 

 as delicate as the finest penciling could make them, are continuous 

 throughout the whole length of the wall examined. (PL VII.) 



The whiter stratum at the base is not sufficiently compacted to allow 

 the collection of a hand specimen. The grayer layers above have the 

 consistency of a soft argillaceous sandstone, but have a very low specific 

 gravity. Between the gray laminated layers and the base of the rhyo- 

 lites is a zoue of peculiar rock that seems to show an imperceptible tran- 

 sition from the one to the other. It is quite impossible to tell just where 

 the rock ceases to be an aggregation of grains and becomes a true crys- 

 talline iava. This transition rock is dark in color, as is also the lower 

 part of the sheet of rhyolite, and at a little distance looks very much 

 like a basalfc. Its weight, however, is not half that of an ordinary 

 basalt, and when examined closely apparently consists of dark and light 

 glossy graios imbedded in a paste of fine gray ash. The coarser lamiua 

 below contain a great deal of more or less finely pulverized sanidin. 



In studying the delicate lamination of these tufaceous deposits one is 

 led to inquire what could have been the method of their deposition. 

 That they are not sub-aerial is evident, for it w^ould be necessary to 

 suppose the entire absence of all winds, as the least irregularity of de- 

 posit wonld destroy the continuity of the laminae. It seems more proba- 

 ble that the almost impalpable dust from distant vents has been cast 

 by the winds om. the surface of deep and quiet waters and been allowed 

 to settle on the bottom beneath. And yet with this exijlanation it is 

 almost incredible that as much as ten feet of such delicate strata could 

 he laid dowit without including a single particle above the size of a pin- 

 liead or witbont the. intrusion of any form of organic body whatever. 



The tafaeeous stratum slopes at first about 1G°, and further south, if 

 it continnes beneath the rhyolite, becomes horizontal. At the point ex- 

 amined, it has !>een weathered out to the depth of several feet, allowing 

 the ihjQlite ledge to overhang. Similar and still more interesting de- 

 posits nnderiie tihe rhyolites of the grand caiion of the Yellowstone, and 

 will be described, in the proper place. 



The rhyoiite of the overlying sheet is quite dark, and by those who 

 liaveseeii it at a distance has been called basalt. It is a dark purplish- 

 gray rhyolite of moderate weight, and is is'o. 2 in Captain Button's 



