U. S. GEOLOGICAL SURVEv 



MONOGRAPH XIX PLATE XXXIV 



Fig. 1. — Muscovitic biotite schist. 



Fig. 2.^Biotite schist 



F,g 3.-B.ot,te slate p,g, 4.-B(ot.t« siate 



THIN SECTIONS SHOWING DEVELOPMENT OF MICA-SCHISTS AND MICA-SLATES. 



