1 88 1. 53 Tram,. N. Y. Ac. Set. 



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 lava 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 sphasrulites, 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 : b, longitudinally axial : c, parallel : d, confused 

 and orderless. The development of fibres is, indeed, a phenomenon very charac- 

 tejistic 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, sphserulites, etc., within them, may otTer another mode of origin 

 for the formation of axiolites and sphasrulites. 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 

 -circumstances 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. Geo!. Expl. 40th Par., VI, Microsc. Petrog., pp. 201-205. 



