Geology and Mineralogy. 67 



diate chemical composition, and consist of biotite-gneisses, biotite- 

 granulites, and, more rarely, biotite schists — rocks in which 

 hornblende is rare or altogether absent, but which, on the other 

 hand, are often remarkably rich in garnets. Neither corundum 

 nor spinel have been certainly detected in these rocks. 



Interfoliated with these ordinary gneissic rocks, which form the 

 great mass of the mountains, we find rocks of much more acid 

 composition, including very coarse pegmatites and graphic 

 granites, aplites and granulites (leptynite or Weiss-stein), granu- 

 lar quartzites, and orthoclase-epidote rocks. The orthoclase of 

 these rocks frequently contains inclusions of fibrolite and other 

 minerals, it often exhibits the " murchisonite " modification and 

 partings, and is not unfrequently converted into "moonstone;" 

 still more complete alterations of the orthoclase into epidote, 

 muscovite, and kaolin being by no means uncommon. In the 

 rubellite district of Nyoungouk these acid rocks contain pink 

 and blue tourmaline (rubellite and indicolite), often beautifully 

 zoned, and it is probably from rocks of this class that the fine 

 gem rubellites are derived. 



Of still greater interest are certain other subordinate rocks of 

 basic and sometimes ultra-basic composition. These include the 

 remarkable pyroxene-gneisses and pyroxene-granulites, which 

 have in recent years been described as occurring in so many 

 widely-scattered regions — such as Ceylon, Southern India, Cen- 

 tral and Southern Europe, Norway and Sweden, Brittany, Spain, 

 Algeria, Eastern, Western, and Southern Africa, the United 

 States and Canada, Brazil, and New Caledonia. In these rocks 

 the feldspars are for the most part basic ones, near to anorthite ; 

 the crystals almost always exhibit the phenomenon described by 

 French petrographers as "quartz of corrosion," and the partial or 

 complete transformation of these feldspars into scapolite (" wern- 

 eritisation ") can frequently be traced. The ferro-magnesian sili- 

 cates are represented by many varieties of augite (sahlite, diopside, 

 and segerine), of enstatite (bronzite and hypersthene), and more 

 rarely of hornblende. Garnets are a frequent and abundant con- 

 stituent in many of these rocks, which in their accessory min- 

 erals and their structures often exhibit many features of striking 

 interest. By the gradual disappearance of the feldspars from 

 these rocks, they pass into remarkable varieties of pyroxenites 

 and amphibolites. The chief varieties of these rocks, which are 

 now described from Burma, are the following : Augite-gneiss 

 (with sahlite, green diopside, etc.), augite-granulites (very rich in 

 garnet), enstatite-gneiss (with bronzite or hypersthene), enstatite- 

 granulites (rich in garnet), scapolite-gneisses, scapolite-granulites, 

 pyroxenites and amphibolites of many varieties, and lapis-lazuli 

 (lazurite-diopside-epidote rock). Many of these rocks contain 

 crystals of calcite scattered through them. 



It is with these basic rocks, and more especially with the ultra- 

 basic types last mentioned, that the remarkable crystalline lime- 

 stones that contain the rubies and spinels are most intimately 



