|
tom,.»v,�a�..,u.. ,. ...............,,,.,..._,. ..«....,«...,,,.,,.,..:.F........ ,..a.•.._».,...,Y.,._,..�..__.,__..,....,.._,.,..,.,_.. .._.�..,_._.., ,_... _. ..,.,...._...-..._. ,........,.._.,.... ,... .,.,,.....,..,.,_, ,...,
<br /> { 2 1042
<br /> I 153
<br /> i
<br /> of a welded tuff. . His chemical, analyses stow that :the davit-
<br /> rification is accompanied, by a loss of Na2O and fit} and, an
<br /> In thin-sao;:�,ozi; . the obsidian has about two pex'c mt of
<br /> ph ocrys'ts of sani4ine -and normally Boned oligoclase in a
<br /> gla. The matrix. � glass has a 'refractive index of 1.50o!
<br /> a
<br /> .003 corresponding to an approximate silica parcentage of
<br /> 72• oxi the curve of. George (14.24) and 71 on the as re�g+af�•
<br /> rr ' silica refractive, index curve of fused glass beads of fiber
<br /> and R.ne art (1966, p. 108). k number of crys ta.l431tcs a:kd
<br /> mi.crol.ites are present in the glass. Perlritic texVire and
<br /> flow-layering are commonly seen in the outcrops. The ob-
<br /> si.dian.zone is approximately 50 feet thick.
<br /> Crass (1.896-, .p. 324) gave= three chemical, analyses of
<br /> specimens fr*m this lava flow, These are reproduced with
<br /> normative compositions calculatdd by tha pracent allttior in
<br /> table- 11. The norms are plotted on a Johannsen ,diagraxn (fi,s.
<br /> 27) and apparently the obsidian i.a; lcall.y, 'a qucrtz-
<br /> latite. and the, two felsites are, chadlea.lLy,, rhyyoli.tes
<br /> and contain..consi.derabl.y more normative ct'tboclase and lass
<br /> normative .a lbi.te and anorthi.te. This di.scr-spe:nvy may be
<br /> explzined b7 a.. loss of soda and a gain. of potash upon devit-
<br /> rificatian- of the obsidian at;niiar .to chaaga.5 described by
<br /> Symons �1.96?_).. Sylch` -Chr.nges can nr-eta son.at' i.cal.ly cause an.,
<br />
|