. Annual Reports of the Department of the Interior for the Fiscal Year Ended June 30, 1900--Twenty-First Annual Report of the United States Geological Society. te lamellae in one of the twinning is not uncommonly combined with this. Particu-larly in the rock from Rattlesnake Hill these pheno- cr3sts contain large and irregular cavities, whoseprincipal extension is related to the crystallographicaxes, such cavities as frequently result from rapidgrowth (see fig. 21). The feldspars of the second gen-eration are laths twinned according to the albite law(in few stripes), and
. Annual Reports of the Department of the Interior for the Fiscal Year Ended June 30, 1900--Twenty-First Annual Report of the United States Geological Society. te lamellae in one of the twinning is not uncommonly combined with this. Particu-larly in the rock from Rattlesnake Hill these pheno- cr3sts contain large and irregular cavities, whoseprincipal extension is related to the crystallographicaxes, such cavities as frequently result from rapidgrowth (see fig. 21). The feldspars of the second gen-eration are laths twinned according to the albite law(in few stripes), and not infrequently according to theCarlsbad law as well. They are, like the cr3^stals ofthe first generation, quite generally fresh, and it wasfound possible to determine both by the methods ofMichel Levy. There seems to be but little differencebetween the composition of the feldspars of the twogenerations, measurement of extinction of albite twins in the sym-metrical zone giving for both varieties double angles as high as 70°.Examination of the Carlsbad twins which showed also albite lamelUe inone of the individuals furnished the following measurements:. Fig. 21.—Form of pari-ties in feldspar ofthe main basalt(magnified). Extinction uhi/Ick m-ar the symmetrical zone of feldspars. 1. 2. 3. -1. Albite 1 19° (Av. ) .y.,0 j (Diff. l( 20° (Av. 23) 26° (Diff. 14 39° (Av. 37)35° (Diff. 23)14° 39° Albite r ... Av. )34° Carlsl)aa 2 (Diff. ° 1 fetude surla d<5termination des feldspaths dans plaques minces, etc., by LC-vy, Paris,1894, 7i NEWARK SYSTEM OF POMPEEAUG VALLEY, CONN. Nos, 1 and 2 of the above table represent the more acid labradorite. of composition near Ab^ Aiii, and Kos. 3 and i the more basic ones, with composition near Ah, An^. Although usually moderately fresh, the feldspars in some specimens contain fine green uralitic and chloritic fibers in fine lines crossing thecrjstal. These lines have a commondirection for th
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