BUILDING AND ORNAMENTAL STONES. 439 



K.— AMPfll BOLE PLAGIOGLASE liOGKS (TRAP AND GREEN. 



STONE IN PART). 



(1) DIORITES. 



Diorite from the Greek word dtoptqsio, to distinguish. 



Diorites are entirely crystalline granular rocks composed essentially 

 of plagioclase and hornblende. 



They are pre-Tertiary eruptive rocks occurring mostly in dikes and 

 intrusive sheets and basic in composition, containing only from 50 to 

 54 per cent, of silica. In structure they are massive. The individual 

 crystals composing the rock are sometimes grouped in globular aggre- 

 gations forming the so-called orbicular dioritc or Jcugel diorite. The 

 texture is as a rule compact, line, and homogeneous, though sometimes 

 porphyritic. The common colors are dark gray or green. According 

 to Zirkel the average composition is : 



Per cent. 



Silica 48. 50 to CO. 88 



Alumina 15. 72 to 22. 12 



Protoxide of iron 6. 20 to 11. 91 



Lime 5.47to 7.99 



Magnesia 0.54to 9.70 



Potash 1.05 to 3.79 



Soda 2.20to 5.21 



Water O.OOto 1.90 



In classification two principal varities are recognized, mica diorite or 

 diorite in which black mica is present in excess of the hornblende, and 

 hornblende diorite or diorite proper. The presence of quartz gives rise 

 to the variety quartz diorite. The name tonalite has been applied by 

 Yom Eath to a quartz diorite containing the feldspar andesite and 

 very rich in black mica and which occurs in the southern Alps. 



Diorites are commonly known by the names trap and greenstone, as 

 are also the diabases. 



These rocks are as a rule exceeding compact and strong, but are 

 scarcely at all used for building purposes owing to their lack of rift and 

 poor working qualities in general. Their somber colors are also a draw- 

 back to any form of architectural display. In England diorites are 

 stated by Hauensclrild* to be largely used for road materials, while the 

 celebrated kugel diorite or napoleonite of Corsica has been abundantly 

 utilized through Italy for interior decorative work. 



Porphyritic diorites, or porphyrites, may be said to bear the same re- 

 lation to true diorites as do the quartz porphyries to granites. That is, 

 they consist of a compact felsitic base in which hornblende or feldspar 

 is porphyritically developed. The celebrated red Egyptian porphyry 

 or u Eosso Antico" is a porphyrite as shown by Delesse.t The source 

 of this rock is stated by this authority to be the Dokhan Mountains, 



* Katechismus der Baumaterialien, p. 81. 

 t Bull. Soo. Geol. do France, 1819-50, p. 524. 



