ARCHAEAN. 



35 



" 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, Central 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 felspars 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 felspars into scapolite 

 (' werneritisation ') can frequently be traced. The ferro-magnesian silicates are 

 represented by many varieties of augite (sahlite, diopside, and aegerine), of 

 enstatite (bronzite and hypersthene), and more rarely of hornblende. Garnets 

 are a frequent and abundant constituent in many of these rocks, which, in 

 their accessory minerals and their structures often exhibit many features 

 of striking interest. By the gradual disappearance of the felspars 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 limestones that contain 

 the rubies and spinels are most intimately associated ; indeed the passage of 

 rocks consisting of various silicates with a few calcite crystals into masses 

 principally composed of calcite, but with the silicate minerals and oxides 

 dispersed through them, is of the most insensible kind. Some of the ruby- 

 bearing limestones are highly micaceous (' cipollinos ') ; others are ' calciphyres,' 

 in some of which the individual calcite crystals attain enormous dimensions. 

 With the rubies and spinels are found a great number of oxides and silicates 

 both original and secondary, with much graphite and pyrrhotite." 



****«*»» 



" The association of minerals in the remarkable crystalline limestones of 

 Burma is worthy of the most careful consideration. Corundum — in its various 

 forms of ruby, sapphire, white sapphire, oriental amethyst, orintal topaz, 

 etc. — is found associated with red, purple, brown, black and other spinels, the 

 relative proportions of the minerals composed of aluminium oxide and of mag- 

 nesium aluminate being very variable. The other minerals present in the 

 crystalline limestones are zircon (rare) ; garnets (abundant in some places) ; 

 a remarkable blue apatite ; felspars, of many species and varieties (including 

 murchisonite, moonstone, sunstone, etc.), and in every stage of alteration ; 

 quartz (in many varieties, and exhibiting some remarkable peculiarities of 

 crystallisation) ; micas (phlogopite, fuchsite, with muscovite and other second- 

 ary and so-called hydro-micas) ; hornblende and arfvedsonite ; augite (sahlite, 

 diopside, and aegerine) ; enstatite (bronzite and hypersthene) ; wollastonite ; 



D 2 



