4:64: Sterrett — Tourmaline from San Diego County, Cat. 



Fig. 10 is the same as !No. 9 of the plate and is a doubly termi- 

 nated crystal, the dotted line representing the development of 

 one of the lower rhombohedral faces. The crystal shown in 

 "fig. 11 was loaned by Mr. Schernikow for examination and is 

 unusual owing to the development of x and t modified by the 

 presence of an o face. One t face (on the upper right hand 

 side) is divided into two parts by a deep striation. Pig. 12 is 

 an end and side view of a section cut from the low T er end of a 

 pale pink crystal with green cap. A natural basal plane c' 

 forms one surface of the section, and is modified by small r' 

 and e' faces. The intensity and thickness of the two colors are 

 nearly equal and an interesting feature is that on looking 

 through the section in the direction of the vertical axis, the 

 two colors complement one another and a nearly white light is 

 transmitted. Another transverse section with an oil-brown 

 core and rose-colored exterior, each of the finest quality, gives 

 a good illustration of the internal strain crystals may be sub- 

 jected to, owing undoubtedly to variations in chemical compo- 

 sition ; for in most positions under the polariscope, it gives a 

 biaxial interference figure. Some crystals have internal stria- 

 tions parallel to the vertical axis, ending in little pits or etch- 

 ings on the terminal faces, and when coarse, often partly filled 



12 



P k. 

 9 r 



c 



with clay. When these striations are fine and numerous, the 

 crystals may be cut " en cabachon," and give most excellent 

 cat's-eye effects. 



In conclusion the writer wishes to express his sincere appre- 

 ciation of the constant assistance and advice of Prof. S. L. 

 Penfield in the preparation of this article ; and also to acknowl- 

 edge the courtesy extended to him by Mr. Schernikow. 



