6o8 



SCIENCE. 



masses of volcanic luffs of widely varying character are 

 dispersed throughout these regions in the West to an ex- 

 tent 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 remarks : * 



" The foregoing descriptions show in what abundance those 

 fibrous bodies in which the fibres are not grouped radially around 

 a centre, as in sphcerolites, but arranged axially along a longi- 

 tudinal line, are disseminated through these rhyolites. .. . These 

 axiolites usually consist of distinct, uniformly thin fibres. r>r of 

 wedge-like par.icles. . . . We see in the arrangemr nt of the fibres 

 in these rhyolites four different types : a, centrally radial : b, long- 

 itudinally axial : c, parallel: d, confused and orderless. The de- 

 velopment of fibres is, indeed, a phenomenon very chataci- listic cf 

 rt.yolites, etc., e:c.' 



A comparison of these facts with those presented ia my 

 examination of these tuffs appear to me significant, net of 

 the development of fibration, etc., in a fused mass, but of 

 the fragmental origin of at least many rhyolytes. obsidi- 

 ans, etc., as suggested in the study of No. 5. The evi- 

 dences 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 microliths, sphcerolites, etc., 

 within them, may offer another mode of crigin for the 

 formation of axiolites and sphcerolites. The anomalous 

 presence of augite in a quartzese reck 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 tufts, rather than to in- 

 digenous 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 thou- 

 sand miles w ide in some places, viz., from the crest of the 

 Sierra Nevada to the eastern foothills of the Rocky Mts., 

 and with a north and south extension of thousands of 

 miles in British Columbia, the United States and Mexico, 

 we have an extraordinary display of the products of vol- 

 canic action. This is the great silver belt of the world, 

 and is also rich in mines of gold, copper, lead, etc. 

 Throughout all the Paleozoic and Mesozoic ages this 

 country was an unbroken though not entirely unwarped 

 sub-marine or sub-aerial plateau, where the most continu- 

 ous and extensive series of sedimentary rocks was depos- 

 ited of which we have any knowledge. At the close ot the 

 Jurassic age the western portion of this region was folded 

 up, to form the great chain of the Siet ra Nevada and Cas- 

 cade Mts., and along this line of fracture numerous vol- 

 canic vents were established. Lassen's Butte, Mt. Shasta, 

 Mt. Hood, Mt. Baker, etc., which have continued in inter- 

 mittent activity to the present day. In Tertiary times 

 the plateau east of the Sierra Nevada was broken up by 

 a series of north and south fractures resulting in the 

 formation of the remarkable system of meridional moun- 

 tain ranges which constitute the chief topographical fea- 

 tures of the district. These mountain ranges are com- 

 posed of blocks of Paleozoic limestones and sandstones 

 --now converted into marbles and quartzites — set up on 

 edge or at a high angle, or of volcanic materials which 

 have welled up through some of the fissures. Along the 

 lines of fractures are great numbers of hot springs, the 

 representatives of thousands more which existed in former 

 days, and to which we owe the great system of fissure 

 veins of this country : — hot water charged with mineral 

 matter gradually depositing this and filling the channels 

 through which it flowed. 



The volcanic rocks which have been poured out in so 



• U. S. Geol. F.xpl. 40th Par., VI, Microsc. Petrog., pp. 201-205. 



many places exhibit a great variety of physical and chemi- 

 cal characters, but have been grouped by Richthofen 

 and Zirkel into five species — prop) lite, rhyolite, trachyte, 

 andesite and basalt. Capt. DUTTON who has given great 

 attention to the volcanic rocks of the West, has distin- 

 guished a larger number of kinds and has adopted a dif- 

 ferent classification. Aside from these massive rocks there 

 is another group which constitutes a marked feature both 

 in the topography and geolcgy, and these are those which 

 have been made the subject of Mr. Julien's paper. They 

 are generally soft in composition, often highly colored, — 

 white, red, blue, green, gray or yellow— more commonly 

 white, red or gray. They are often quite local and usu- 

 ally occupy the lowlands, frequently underlying much of 

 the level surface between the mountain ranges ; and their 

 best exposures are seen in the banks of streams which 

 have cut these lowlands. There they are shown to be often 

 horizontally bedded and sometimes interstratified with 

 lacustrine sediments and sheets of basalt. Typical expo- 

 sures of these rocks may be seen at Eureka, Nevada, 

 where houses and cellars are excavated in the soft material 

 which forms the sides of the valley; at Challis, in the 

 banks of Salmon River and Garden Creek, whence the 

 specimens described by Mr. Julien came, and in the 

 canons of the Des Chutes and its tributaries in Oregon. 



Economicallv these rocks have-considerable inportance 

 as they are extensively used in place of fire brick for lin- 

 ing lead smelting furnaces, being very refractory, and 

 easily dressed into shape with an eld axe. 



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 14 



The above sectioD represents the filling of some of the fresh water lakes 

 which formerly existed in Oregon just east of the great volcanic cones of 

 the Cascade Mountains. Numbers i and 11 represent sheets of basalt, 

 the even number softer tuffs and bed of diatomaceous earth, the odd num- 

 bers consolidated conglomerates of volcanic materials called "concrete'" 

 in my notes. 



The study of a large number of outcrops of this series 

 of rocks lrom Southern Arizona to the Columbia River 

 has convinced me that they are generally volcanic ashes 

 which have been washed down and more or less per- 

 fectly stratified in bodies of water which formerly occu- 

 pied the intervals betsveen the mountain ranges of the 

 great basin. On the Des Chutes a section of more than 

 1000 feet shows 25 alternations of strata, many of which are 

 examples of the rocks in question. Here they are inter- 

 stratified with beds of tripoli, composed of fresh water 

 diatoms, and layers of basalt. Some of the ash beds are 

 almost entirely composed of lapillae of soft cottony 

 pumice, others are finer, grey, red, white, etc., and con- 

 tain the trunks ot coniferous trees, and in some instances 



