BUILDING AND ORNAMENTAL STONIOS. 439 



K.— AMPIllBOLE PLAGIOCLASE KOCKS (TKAP AND (UIEION. 



STONE IN rAliT). 



(1) DIOKITES. 



Diorite from the Greek word ocop::;-:cu, to distinguisli. 



Diorites are entirely crystalline granular rocks coni{)osed essentially 

 of plngio(;lase and liornblende. 



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

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

 54 i)er cent, of silica. In structure tliey are massive. The individual 

 crystals composing the rock are sometimes grouped in globnhir aggre- 

 gations forming the so-called orbicular diorite or laujcl diorite. The 

 texture is as a rule comi)act, fine, and homogeneous, though sometimes 

 porphyritic. The connnon colors are daik gray or green. iVccording 

 to Zirkel the average composition is: 



I'd- Cl'lltr. 



Silica IS. no to GO. 88 



Aluuiiua IT). 72 to iJ:i. 1 ii 



Protoxide of iron (>. LMi lo 1 1. ;» j 



Lime r>. 47 to 7. '.»'.) 



JMagnesiii 0. M to 11.70 



Potixsli 1.0.") to :;. 70 



Soda 2. 20 to r,. 21 



Water 0.1)0 to l.'JO 



In classilication two i)rinci[)al varities are recognized, miea- diorite or 

 diorite in wliieh black mica is present in excess of the hornblende, ;iiid 

 hornblende diorite or diorite projjcr. The presence of (juartz gives rise 

 to the variety (juartz diorite. The name tonalite has been a[)plie(l by 

 YoMi IJath to a (juartz diorite containing the feldsi)ar audesite and 

 very rich in black mica and which occurs in the soutliei'u 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 ])ui'i)Oses owing to their lack of rift and 

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

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

 stated by Ilanenschild* 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. 



Por[)h3'ritic diorites, or ])orphyrites, maybe said to Ix'ar I lie sau)e le- 

 lation to true diorites as do the (juartz ])orphyrics to granites. That is, 

 they consist of a (;oini)act felsitic base in which hornblende or felds[)ar 

 is i)orphyritically devel()j)ed. The celebralcd J'cd f^gyptian porphyry 

 or "llosso ATitico" is a ])orphyrite as shown l)y Dehisse.f The source 

 of this rock is stated by this authority to be tlu; Dokhan iMonidains, 



* Katocliisimin der |}atimaLerinli(!ii, p. 81. 

 i Bull. Soc. Gcol. do Friiucc, ISlD-jO, \\. 524. 



