Geology and Mineralogy. 69 



occur in the limestone, and this limestone is of a very remarkable 

 character. There are no facts which point to the conclusion that 

 the limestone was originally of organic origin, but many circum- 

 stances suggest that it may have been formed by purely chem- 

 ical processes going on at great depths within the earth's crust. 

 The highly-crystalline calcareous rock, besides containing so 

 many silicates and oxides, is associated in the most intimate 

 manner with pyroxene-gneisses and granulites containing anor- 

 thite, and with various pyroxenites and amphibolites. The lime 

 feldspars and lime-soda feldspars of these rocks show the greatest 

 tendency to undergo change — passing into scapolites by the 

 process known as " werneritisation," and eventually giving rise 

 to the separation of calcium carbonate and hydrated aluminium 

 silicates. That from the last-mentioned salts the hydrated oxides 

 of aluminium (diaspore, gibbsite, bauxite, etc.) may be separated 

 has been shown by the studies of Liebrich and others, while the 

 conversion of these substances into the anhydrous aluminium 

 oxide has been shown to take place by H. St. Claire Deville, 

 Stanislas Meunier, and others. 



Crystallized aluminium oxide (corundum) has now been formed 

 by chemists by no less than 20 different processes, and in some 

 cases, like those described by Senarmont, Weinschenk, Bruhns, 

 and Friedel, the formation and crystallization of the substance 

 has been effected at very moderate temperatures under pressure. 

 By one or other of these or similar methods it is probable that 

 the formation of the Burma corundum and spinel has been 

 effected, the source of the minerals being the decomposition 

 products of basic and easily-altered lime feldspars in the pyroxene- 

 gneisses. 



Of still greater interest than the question of the origin of the 

 corundums and spinels are the problems connected with the 

 remarkable changes that these minerals undergo in deep-seated 

 rock masses. The rubies of Burma, when found in situ in the 

 limestones, are usually seen to be enveloped in a mass of materials 

 produced by the alteration of their superficial portions. Nearest 

 to the unaltered gem is a zone of diaspore — the hydrated 

 aluminium oxide— and this is found to pass insensibly into various 

 hydrous aluminous silicates — margarites and other clintonites, 

 vermiculites, muscovites, kaolinites, etc. While, in some instances, 

 the corrosion of the rubies appears to have gone on in a seem- 

 ingly irregular manner, in the majority of cases a very definite 

 mode of metamorphosis may be detected by the study of the 

 various examples. There are evidently certain planes of "chemi- 

 cal weakness " (analogous to the cleavage planes, gliding planes, 

 and other directions of physical weakness) along which decompo- 

 sition goes on most readily. The principal of these solution 

 planes is the basal plane, and parallel to it we find the gems eaten 

 away in a series of step-like surfaces. Other less pronounced 

 ^^planes of chemical weakness exist parallel to the prism faces. 

 Unaltered corundum is, like quartz, destitute of true cleavage, 



