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T 1042 <br /> i 154 <br /> • <br /> original quartz-Iratite vitr©phyre to become a rhyr'lite fel- <br /> sit ... Differences between the che&:LcaI Compositibn'bf th'u <br /> felsites -(samples, 1,5 'and -16 table' 11 ) and the obsidian <br /> (sample 14s table '10 ,are the same as campositian I differ- <br /> erices, Mug :d .by ' : 'ao:nda -y, de'vi r f catiari of a°s y� o canic <br /> rocks. Lipman (1965, p. 19-21) who studied paired sample$ <br /> Of glassy and crystalline voleanic mocks, found ,that in five <br /> . rout. of six pairs there i a gain of Sio2 and R20 and a loss <br /> of H20, Ca© and Na20 with no change of Al2©S. Similar <br /> compositional di.ffozences are prssent. at Silver." Cliff. <br /> H1 r tz La- . <br /> This unit (Tql on plate 3) accurs as a sill an,d dikes <br /> w'hich 'intrude rhyolite brecci.a. in N�j sec, 89 T. 22 s. r R# <br /> 72 W. and rhyoli.te lava in SWk, Ssj4, sec. 15, T. 22 s. 3� R. <br /> 72 W. In the first locality 'a sill is poorly exposed :axed <br /> has -been'm$pped largely_ by tracing float. Its ,contacts are . <br /> not exposed, ind its' thickness is not known, although it is.- <br /> . unlikely to be more: than 40 feet thick. The sill occutifes <br /> a ,seemingly constant, a'tratl.graphie posttign between th;b <br /> rhyol.i.'te tuff, breccl,a.,, :and conglomerate Unit amd the rhyo- <br /> lito br ccl a Tinit. St ; has the appearance of an.-extrusive <br /> bedded ufii.t and would•have been so mapped but for an :eight- <br /> f6o.t-"Ilwide diko 6f identical, petrologic features, trending <br /> N 50. E which is cleaily seen to: intrude the oVerl.yi.ng. <br />