REPORT OF THE CHIEF ASTRONOMER 529 



SESSIONAL PAPER No. 25a 



All of the purely volcanic constituents are andesitic. Conformably overlying 

 these rocks and underlain by other andesitic flows and breccias, comes a widely 

 extended layer of white to pale-gray trachytic or rhyolitic tuff, aggregating 

 perhaps 200 feet in thickness. The top of the whole group is not exposed in the 

 Boundary belt and the series remains incomplete. 



The following table gives an extremely crude idea of the general relations 

 and thicknesses as estimated in the field: — - 



Top, erosion-surface. 

 l,00O-i-feet. — Andesite flows and breccias. 



20Cbt " Liparitic (?) tuff. 



900h- " Andesite flows, ash and breccias. 



100 " Conglomerate. 

 3,000;+^ " Andesite breccias, flows and ash-beds. 



5,200z+rfeet. 



Base, unconformable contact with Custer batholith and Hozomeen sediments. 



The andesitic members are always very massive. It is seldom possible to 

 distinguish the contacts between different flows, and even the contacts between 

 flow and breccia are generally obscure. The individual flows seem to be usually 

 very thick ; cliffy slopes as much as 300 or 400 feet high do not disclose undoubted 

 breaks in the massive lava. 



The more basic material of the breccias, ash-beds, and flows has great uni- 

 formity of composition. Nine typical specimens were collected in different parts 

 of the area and at various horizons from near the base upward. Thin sections 

 of all the specimens were studied. Though not crushed they are all more or less 

 altered. Without exception, the flows and lava-fragments of the agglomerates 

 seem to belong to the one common type, augite andesite. The phenocrysts are 

 regularly labradorite, averaging Ab 2 An 3 , and augite. The latter is generally 

 uralitized pretty thoroughly. Neither primary hornblende nor olivine was 

 detected, though in some cases the former may have accompanied the augite as 

 a subordinate phenocryst. The ground-mass is more altered than the pheno- 

 crysts and is a mass of chlorite, uralite, plagioclase microlites, and indetermin- 

 able material, perhaps derived from glass, which was apparently a very-abundant, 

 staple constituent of the ground-mass. 



The beds of agglomerate are usually thick, individual ones measuring more 

 than 200 feet in thickness. The blocks" are of all sizes up to those a foot or 

 more in diameter. At many points angular fragments of cherty quartzite and 

 slate, identical in look with the dominant rocks of the Hozomeen series, were 

 found in the breccias. At one section in the deep valley northwest of Monument 

 68, and about 1,200 yards from the monument, there occurs a layer of breccia 

 wholly or almost wholly made up of fragments of cherty quartzite and serpen- 

 tine; this bed is at least 75 feet thick. The fragments are angular, ranging 

 in size from sand-grains to blocks six inches or more in diameter. There can 

 be little doubt that these fragments were derived from the Hozomeen series. 

 The bed shows no sign of water-action; from its position in the midst of 

 manifest volcanic agglomerates, it may best be regarded as a special product 



25a — vol. ii — 34 



