530 DEPARTMENT OF FEE INTERIOR 



2 GEORGE V., A. 1912 



of gas-explosion which operated in this vicinity and blew out a large quantity 

 of the foundation rock. The matrix of this bed was not examined microscopi- 

 cally; it may be a fine andesitic ash. 



Sometimes, though rarely, granitic and gneissic blocks appear in the staple 

 breccias; most of those observed seem to have been derived from the underlying 

 Custer batholith. 



About one mile northwest of Monument 68 a bed of coarse conglomerate, 

 100 feet or more in thickness, interrupts the succession of breccias and flows. 

 The pebbles are very well rounded and were unquestionably long rolled by waves 

 or currents. They vary in size but few are over a foot in diameter. 

 They consist of altered andesite (dominant kind), quartzite, chert, slate, and, 

 rarely weathered granite. The matrix is sandy. The bed dips about 16° to 

 the eastward and seems to be quite conformable to the yet more massive volcanic 

 members above and below. 



Above the conglomerate the tuffaceous rocks carry several thin conformable 

 lenses of gray argillite, which also appear to have been laid down under water. 



The acid tuff was seen at two localities. It crops out on the summit of the 

 rugged ridge 1-5 miles south of Monument 68 and on a much greater scale 

 upon the long ridge running eastward from Monument 69. This tuff covers 

 the latter ridge for one mile of its length and from its white colour is very 

 conspicuous in the landscape. The tuff is extremely jointed, so that it is 

 difficult to secure a hand-specimen of standard size. Some of the rock is vesi- 

 cular and it is possible that thin flows are represented in the middle part of 

 the 200-foot band. The whole composite mass overlies the andesites, dipping 

 at angles of from 10° to 15° to the north. On the higher ridge on the west the 

 acid tuff seems to be overlain by younger andesites roughly estimated to be 

 1,000 feet thick. 



The acid tuff is nearly pure white to pale-gray when fresh, weathering white 

 to pale-yellow. Macroscopically it is quite aphanitic for the most part, with 

 only the rarest suggestion of a small feldspar phenocryst. The rock reminds one 

 of porcelain viewed on a broken edge. Under the microscope the phenocrysts 

 of the angular fragments are seen to be few in number and to have the proper- 

 ties of sanidine or orthoclase. The ground-mass is a cryptocrystalline aggregate 

 of quartz and feldspar, with the character of a devitrified obsidian. The matrix 

 of the tuff is optically like the ground-mass of the fragments. The mass has 

 clearly the composition of an acid obsidian and is perhaps nearer trachyte 

 than rbyolite. 



The age of the formation has not been determined by direct fossil evidence. 

 The lava-flows, ash-beds, breccias, and interbedded conglomerates are not 

 crushed. The dips are generally low, running from 5° to 30° as the observed 

 maximum. The breccias and conglomeratic beds contain many fragments and 

 pebbles of quartzite, slate, and granite which were without much doubt derived 

 from the eroded Hozomeen series and the Custer gneissic batholith. It seems 

 reasonably certain, therefore, that the vulcanism dates from a period much later 

 than the intrusion of the batholith and, a. fortiori, than the folding of the Hozo- 



