302 



University of California. 



[Vol. 



It was first noticed in the form of white crystals projecting with 

 rough, pitted surfaces and irregular outlines from weathered blocks 

 of the schist, the roughness of the surface and the lack of sharp 

 crystal boundaries being due to the presence of abundant inclusions 

 of the various other component minerals of the schist. Besides 

 occurring as a rock constituent, lawsonite is found in much larger 

 crystals, generally free from noticeable inclusions, embedded in a 

 greenish-white micaceous mineral, determined as margarite, in 

 veins traversing the schist, and also lining or filling smaller veins 

 and cavities, as aggregates of clear colorless crystals, associated 

 with actinolite in delicate acicular tufts. 



Crystals of lawsonite are of simple habit, the most conspicuous 

 faces being those of the prism, basal pinacoid, and brachydome. 

 Those which project freely into cavities have the general form shown 

 in Plate 17, Fig. 1; those embedded in the margarite, the tabular, 

 and extended habits of Figs. 5, 6, and 7. A thin section of the 

 massive portion of the schist shows that the mineral encloses the 

 glaucophane and other constituents, as ice in a pond imprisons the 

 sticks and grains at its margins. Yet it retains somewhat its idio- 

 morphic form, as appears when the rock is subjected to weathering. 

 The structure under the microscope is similar to the micropoikilitic 

 structure, but differs, of course, genetically, from the structure in 

 igneous rocks to which that term has been applied.* 



CRYSTAL FOKM. 



Lawsonite crystallizes in the orthorhombic system. The axial 

 ratios a:b:c = .6652 : 1 : .7385 were calculated from the following 

 angles, measured on a Fuess Universalapparat : — 



graphical study of some of the schists from the Tiburon Peninsula, at the 

 University of Munich, and upon accidentally learning through correspondence 

 that the work embodied in the present paper was well under way, he imme- 

 diately placed his observations unconditionally at the service of the writer. 

 Such results as he was able to arrive at, with the very limited material at his 

 disposal, and his suggestions, have been of great service in the preparation of 

 this paper. It is gratifying to note that, in the matter of choosing a name, 

 Dr. Palache and the writer are in perfect accord, as lawsonite was the entirely 

 independent choice of both. 



*G. H. Williams, Jour. Geol., Vol. I, No. 2, 1893. 



