196 SMITH— PARAGENESIS OF MINERALS. [Octobers, 



faces observed in the rocks, the prism faces meeting at an angle 

 of 67° 16'; consequently longitudinal sections usually give rectan- 

 gular outlines, and basal sections rhombs. There are three cleav- 

 ages, that parallel to the basal pinacoid, and that parallel to the 

 brachypinacoid being rather distinct, while the cleavage parallel to 

 the prism is faint. In longitudinal sections the pinacoidal and basal 

 cleavages are at right angles to each other, while in basal sections 

 the distinct brachypinacoidal cleavage bisects the obtuse angle (112° 

 44') of the faint prismatic cleavage. The plane of the optical axis 

 lies in the brachypinacoid (010) and the acute bisectrix is the axis 

 of least elasticity, and is the normal of the basal plane. The charac- 

 ter of the double refraction is therefore positive. Sections parallel to 

 the vertical axis show high double refraction, and in thin slides show 

 second order interference colors, while basal sections give weak 

 double refraction and in thin slides show no higher color than yellow 

 or orange of the first order. The orientation of the optical figure 

 with reference to the cleavages and the axes of elasticity affords a 

 ready means of identifying this mineral in microscopic sections. The 

 relief is high and in thin sections the mineral is colorless, though in 

 thicker sections and in macroscopic crystals the color is light blue. 

 These characters make the determination of the mineral easy, even 

 in minute crystals. 



Two analyses of lawsonite are quoted below, both made of the 

 mineral from the original locality. No analyses of this mineral 



Analyses of Lawsonite from the Tiburon Peninsula. 



^ I. II. 



SiOs 38.10 38.45 



AI2O3 28.88 31.35 



Fe^Os 0.85 ■ 0.86 



FeO o.io 



MgO 0.23 0.17 



CaO 18.26 17.52 



Na20 0.65 0.06 



K2O 0.23 



TiO. 0.38 



H2O (above 100° C.) 11.47 ii-2i 



MnO trace 



Total 98.39 100.33 



I. F. L. Ransome, Bull. Dept. Geol. Univ. of California, Vol. L, 307. 

 IL Schaller and Hillebrand, Amer. Jour. Set. (1904^ p. 197- 



