130 



VOLCANIC ROCKS OP SCHEMNITZ, ETC 



Upper 

 Galleey. 



— «. + ^ lch the ^ *™ ^Powd, from the including mass during 

 Wali^sd. J p pr °? eSS f ^ Sl0w <?*** and, possibly, is sometimes 



the result of the remelting of older lavas, into the comp 

 sition of whieh they entered ; in such cases, if l ess fusibl 

 than the more modern lava, they would be caught up byk 

 and retained, undergoing but little change themselves 



Nos. 42 and 42a are specimens of native sulphur, a mineral 

 frequently found sublimed in and around the craters of vol 

 canos. The sulphur from the Sandwich Islands is chiefly 

 remarkable for containing an admixture of Selenium to 

 which it owes the red colour that is more especially to' be 

 observed on 42a. J e 



Shelf 4 and part of shelf 5 (43 to 93) confab 

 yf the trachytes and associated volcanic rocks of 



of Schemnitz, in H\ 



Of these, 44 to 47 are 



basaltic rocks. 48 to 55 are varieties of pearlstone, some of 

 which (52 and 53) contain sphasrulites. The formation of 

 the latter is owing to the separation of certain parts from 

 the general mass while passing from the melted into the 

 solid or stony state, and is dependent upon the condition 

 under which the cooling of the mass took place. 57 to 68 

 are varieties of greenstone ; the first contains spherical 

 concretions, and 66 to 68, which contain included crystals 

 of other minerals, are known by the name of Porphyries. 



Trachytes are represented by specimens 70 and 85. Of 

 these eruptive rocks, there are no true representatives in 

 the British Islands. The trachytes are, for the most part, 

 less fusible than the ordinary augitic lava, into the com- 

 position of which silicate of lime enters largely, being 

 formed chiefly of orthoclase, or potash felspar. These rocks 

 derive their name from rpaxlf (rough), from the rough, 

 uneven character of their fractured surface. 



94 to 101 are from the Bannat and Croatia, principally to il- 

 lustrate the rocks, associated with the ores, which are worked 

 at Drey Konig mine. Ofthese, 94 to 96 are volcanic, 97 and 

 98 are the rocks associated with and containing the ores. 





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