§ 31. PRECIOUS STONES. 86° 



hardened shale, and finally into hornstone, jasper, and 

 analcime rock containing garnets and copper ore, from an 

 intrusion of greenstone porphyry.* 



In the Ural mountains, which form the dividing crest 

 that separates the waters of Europe from those of Asia, the 

 effect of metamorphic action is strikingly displayed. Sir 

 Roderick Murchison emphatically remarks, that the crys- 

 talline rocks which form the axis of the anticlinal of the 

 Ural chain are for the most part altered Silurian strata. 

 " In the short space of a mile, you may walk upon the 

 edges of the partially altered beds of grit and schist, until 

 you find them converted into amorphous quartz rock, in 

 contact with highly crystalline greenstone ; a rock which 

 is admitted to be of igneous and intrusive character. 

 Coralline limestone is changed into white and green 

 marble. 1 ' f The intense plutonic action which effected the 

 disturbance of the rocks of the Urals, has clearly been the 

 cause of the rich mineral productions of those regions, the 

 metallic veins, and the mineralization and metamorphism 

 of the sedimentary strata. J 



31. Precious Stones. — Connected with the changes to 

 which the metamorphic rocks have been subjected is the 

 formation of some of those minerals, which, from their 

 beauty, splendour, and use as ornaments, are termed pre- 

 cious stones. The Sapphire and the Oriental ruby, or 

 red sapphire, which are prized next to the Diamond, and 

 almost equal that gem in hardness, are found in trap rocks ; 

 and the common Corundum, which is a species of the 

 same mineral, and the Emerald, occur in granite. The 

 sapphire and ruby are pure alumina crystallized ;§ and the 



* Professor Henslow ; Transactions of the Philos. Soc. of Cambridge. 



f Geology of Russia, p. 357. X Ibid. 



§ The sapphire affords, by analysis, 98 '5 of alumina, 0-5 of lime, 

 and 1 of oxide of iron ; the ruby, 90 of alumina, 7 of silex, and 1*2 of 

 oxide of iron. — Phillips's Mineralogy. 



