Annual report of the United States Geological Survey to the Secretary of the Interior . te, iron oxide,and a zircon-liko crystal were noted. Iron disulphide in irregular grains andin one case with crystal form (a parallelopipedon), epidote, and chlorite arealso present. It will be noted that No. 142.» is not from the Mokelumne River area,but from a point many miles to the southeast. This smaller area is with,out doubt of the same origin as that of the Mokelutune River. At manyother points there are dark inclusions in the hornhlcudic granite,which are believed by the writer to be included fragm
Annual report of the United States Geological Survey to the Secretary of the Interior . te, iron oxide,and a zircon-liko crystal were noted. Iron disulphide in irregular grains andin one case with crystal form (a parallelopipedon), epidote, and chlorite arealso present. It will be noted that No. 142.» is not from the Mokelumne River area,but from a point many miles to the southeast. This smaller area is with,out doubt of the same origin as that of the Mokelutune River. At manyother points there are dark inclusions in the hornhlcudic granite,which are believed by the writer to be included fragments of are the basic inclusions in the granite west of Granite Lake inTuolumne County, a photoengraving representing which maybe foundin the previous The wiiter has sometimes referred to theseinclusions as schliere, but this is an error. They do not grade overinto the granite, but are certainly inclusions, whatever their origin. About one half mile upstream from the month of Rear Creek, in the 1 Fourtvcutli Ann. Kept. I*. S. Geol. Survpy. lurt II, p. (8&. a JURASSIC DIORITE-GNEISS. TURNER.] GNEISSES. 705 gneiss series in the bed of the Mokelumne, is a layer of a light-buff rockof medium grain, apparently largely made up of quartz grains withrusty spots and specks of pyrite. In the field this was supposed to bea quartzite, and to give evidence of the original sedimentary nature ofa portion of the gneiss series. The microscope shows tbis rock to bemade up of interlocking grains of quartz, monoclinic pyroxene, andgarnet, with scattered grains of iron pyrite. There is nothing in thestructure of the rock to preclude a sedimentary origin, but the difficultyin recognizing the original nature of a quartz rock may be inferredfrom an examination of PL XXXVII, Fig. B (Xo. 54), and PL XXXVIII,Fig. A (Xo. 183) and Fig. B (Xo. 91)8). In No. 54 the clastic structureis very evident in a portion of the section, and in another part therehas been seco
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