bascom.] APORHYOLTTES. 43 
also sometimes very vesicular. In the latter case the amygdules of 
dark-green epidote and clear quartz, elongated by fluxion and con- 
spicuous against a pale-pink background, render the rock strikingly 
handsome. Phenocrysts are usually present, but generally incon- 
spicuous. 
Delicate lines of flow structure, which are brought out in great 
detail by weathering or are painted in rich colors on the material 
washed by the mountain brooks, are another marked feature of the 
aporhyolites. These liow lines are frequently very sinuous, showing con- 
tortion and crumpling of the lava. (Pis. VII and VIII.) The tlowage 
is often emphasized by the mingling of two contrasting magmas, form- 
ing taxites of either a> eutaxitic or an ataxitic character. A fuller 
description of these taxites is given on page' 57. 
Another characteristic which these aporhyolites possess in common 
with their modern analogues is the spherulitic structure. Spherulites 
are rarely, if ever, altogether absent, and in some localities they are 
crowded so close together as to constitute the major part of the rock 
mass. They range in size from microscopic dimensions to those of a 
butternut. Where there is no regularity of arrangement and they are 
brought out in relief by weathering, the rock has a superficial resem- 
blance to a conglomerate composed of rounded pebbles of remarkably 
uniform size (BB shot) and shape. (PI. IX.) The rich grays and blues 
and purples of the spherulites and matrix render this a conspicuous 
rock. Other specimens from the same locality (the south flank of the 
mountain northeast of the junction of Copper Pain and Toms Creek) 
show spherulites elongated by flow. They are thus drawn out into 
solid cylinders with a diameter of l mm and a length of some 2 em . 
Specimens of aporhyolites composed of spherulites about the size 
and shape of almonds (18 mni by 9 mm ) were also found. The rock had 
been greatly sheared and the spherulites flattened. 
Spherulites become a still more striking feature of the aporhyolites 
when arranged in layers which traverse the face of the rock in long, 
parallel, dotted bands. This arrangement has been described by 
Iddings in the obsidian of the Yellowstone National Park. 1 
While sometimes these bands are 4 mm wide, at a nearly uniform dis- 
tance apart, and of an indefinite length, in other cases they are very 
narrow, dwindling into mere lines and dying out, to be succeeded by 
other lenticular bands. (PI. X.) The planes of these spherulites have 
become planes of weakness and solution. The rock cleaves readily 
parallel to them and shows a coating of secondary silica on the cleav- 
age surfaces. This deposition of silica causes these planes to become 
the hardest part of the rock, and hence they often stand out as 
parallel ridges on the weathered surfaces. 
At the locality mentioned on pages 41-42, piedmontite, in radiating 
needles, associated rarely with scheelite, has, together with quartz, been 
'Obsidian Clin': Seventh Ann. Rept. U. S. Geol. Survey, 1888, p. 276, PL XVIII. 
