1881. 53 Trans. N. Y. Ac. Sez. 
In the thin section the constituents are very much the same as in 
No. 8, with the exception of hornblende, and all the grains are in large 
part rounded. A few elongated rounded grains of a basaltic Java are 
also included, highly microcrystalline with minute ledge of plagioclase 
scattered through a reddish-brown opaque base. 
This specimen, and perhaps the preceding, represent the basic 
division of the tuffs, being ejections from an eruption of basaltic lava, 
though naturally composed of its more fluid, glassy, and acid scoria. 
From these facts it may be concluded that enormous masses of vol- 
canic tuffs of widely varying character are dispersed throughout these 
regions in the West, to an extent which could hardly be appreciated 
from the meagre references in our present petrographical literature. 
In his discussion of the rhyolytes of the fortieth parallel, Zirkel re- 
marks :* 
“The foregoing descriptions show in what abundance those fibrous bodies in 
which the fibres are not grouped radially around a centre, as in sphzerulites, but 
-arranged axially along a longitudinal line, are disseminated through these rhyolites 
These axiolites usually consist of distinct, uniformly thin fibres, or of wedge- 
like particles.... We see in the arrangement of the fibres in these rhyolites four 
‘different types: a, centrally radial: 4, longitudinally axial: c, parallel: d, confused 
and orderless. The development of fibres is, indeed, a phenomenon very charac- 
teristic of rhyolites, etc., etc.” 
A comparison of these facts with those presented in my examination 
of these tuffs, appears to me significant, not of the development of fibra- 
tion, etc., in a fused mass, but of the fragmental origin of at least many 
rhyolytes, obsidians, etc., as suggested in the study of No. 5. The 
evidences of the hot and plastic condition of the fibres and drops of 
volcanic glass, with the occasional exception of a cooled outer shell, for 
a long time after their fall, and of a tendency to the growth of micro- 
liths, sphzerulites, etc., within them, may offer another mode of origin 
for the formation of axiolites and sphzrulites. The anomalous pres- 
ence of augite in a quartzose rock like rhyolyte, to which Zirkel calls 
attention in the same passage, may also find explanation in the varied 
intermixture of minerals which prevails in many tuffs, rather than by 
indigenous development within an acid lava. 
Dr. NEWBERRY said that he had no doubt that Mr. Julien was quite 
correct in regard to the genesis of the peculiar rocks which he had 
described. He had collected the specimens and was able to supply 
some facts in regard to their mode of occurrence. They belong to a 
series of rocks, plainly volcanic, but of which the history has not been 
given by those who have studied the volcanic rocks of the West. The 
<ircumstances of their occurrence are briefly these: over a great belt 
not less than one thousand miles wide in some places, viz., from the 
* U.S. Geol. Expl. 40th Par., VI, Microsc. Petrog., pp. 201-205. 
