240 GEOLOGICAL DESCRIPTION* OF THE EARTH*S CRUST. 



granite covers transition slate for a space of four 

 and penetrates it from above downwards in narrow 

 branching veins, having wedge-shaped terminations. ( 252 ) 

 These details have been introduced for the purpose of 

 indicating by examples the character of erupted rocks, as 

 shewn in a rock most generally distributed throughout 

 all parts of the earth. As granite covers argillaceous 

 schists in Siberia, and in the Departement de Pinisterre 

 (He de Mihau), so does it cover oolitic limestone in 

 the mountains of Oisons (Fermonts), and syenite and 

 chalk in Saxony near Weinbohla. ( 253 ) At Mursinsk, 

 in the Oural, the granite is porous, and as in the later 

 volcanic rocks, the cavities are filled with magnificent 

 crystals, particularly beryls and topazes. 



2. Quartzose porphyry, frequently imbedded as veing 

 in other rocks. The matrix is usually a fine-grained mix- 

 ture of the same elements as those which form the larger 

 disseminated crystals. In granitic porphyry, which is 

 very poor in quartz, the feldspathic base is almost 

 granular, and laminated. ( 254 ) 



3. Greenstone, Diorite, granular mixtures of white 

 albite and dark-green hornblende, forming dioritic por- 



phyry when the crystals of albite are disseminated in a 

 compact paste. The greenstones, either pure, or, as in the 

 Fichtelgebirge, containing laminae of diallage, and passing 

 into serpentine, have sometimes penetrated, in the form of 

 beds, between ancient strata of green argillaceous schist; 

 but they more often traverse the rock in the form of 

 veins, or appear as domes of greenstone, analogous to 

 domes of basalt and of porphyry. ( 255 ) 



