( 57 ^ 



cavities in crystalline limestone at Welimada. Doubly terminated 

 crystals not rare in gera washings. 



Fig. 4 shows the ordinary hexagonal prisms of quartz. 



Fig. i. — Quartz hexagonal prisms. 



Ghert, SiOg. — A.n opaline chalcedonic rock usually replacing 

 crystalline limestone. 



Chalcedony, SiOg. 



Opal, SiOa + HgO. — In some cases a rock consisting entirely of 

 common opal is found with the less pure cherty varieties. 



Psilomelane, MnO + MnOaHgO.— Commonly associated with 

 limonite and hematite in veins and aggregates resulting from the 

 decomposition of the grannlites. 



Spinel, (MgFe)0Al203. — Blue, green, and red spinels (especially 

 the latter, known as Balas rnby) are used as gems. Small well- 

 formed octahedra (see Fig. 1, page 5:3) of spinel, usually pink, are 

 common in the crystalline limestones. Green spinel occasionally 

 occurs in grannlites, and frequently in the heavy dark green 

 rocks associated with junctions of limestone and granulite. The 

 gem spinels are obtained from the gravels, but are probably 

 derived from the crystalline limestones. 



Magnetite, FeOFeaOg. — Common as an accessory mineral in the 

 granulites ; also in limestones. 



Chromite, FeOCrgOg. 



Chrysoheryl, BeOAlgOg. — Includes cat's-eye and alexandrite. 

 This important gem stone sometimes occurs in large individuals, 

 exhibiting characteristic twinnirg. The beautiful chatoyance of 

 the cat's-eye is perhaps due to the presence of fine tubular cavities 

 arranged symmetrically in the crystal. (The much less valuable 



I 105-04 



