MINERAL VARIETIES OF SYENITE. 223 



products. And when syenite forms dykes, it contains no titanite and 

 less plagioclase than massive syenite. As in trachytes, of which it 

 is a deep-seated representative, its orthoclase is rich in soda. The 

 varieties of hornblende syenite diverge chiefly towards hornblendic 

 granite and granitite in the Vosges, towards quartz diorite in the 

 Odenwald and Ascheffenburg ; while at Laurvig the rock is inter- 

 mediate between syenite and gabbro, containing olivine, and apparently 

 diallage and hypersthene. 



Mica Syenite. Mica syenite is often termed minette. It usually 

 occurs in dykes. It consists of a fine-grained ground mass of ortho- 

 clase, with biotite. The felspar is rarely fresh, it is white or red, and 

 has assumed a microcrystalline texture. Plagioclase is nearly always 

 absent. Though the mica is usually brown, it is sometimes green, 

 or the colours are combined in the same film ; the crystals are six- 

 sided. Apatite, magnetite, and pyrites are met with. Calcite and 

 quartz occur as decomposition products. The best types of mica-syenite 

 occur in Calabria, in the Vosges, and Odenwald. At Framont this 

 rock contains green hornblende. At Wackenbach it contains blue 

 glaucophane. In Lower Alsace the minettes contain augite, which is 

 sometimes altered into chlorite or delessite. The minettes of the 

 Southern Schwarzwald are rich in augite. Titanite is usually absent 

 from mica-syenites. 



Augite Syenite. This rock consists of orthoclase, plagioclase, 

 and augite, with some titanite, hornblende, biotite, pyrites, magnetite, 

 and apatite. The two felspars vary in relative quantity. The plagio- 

 clase often has a glassy character. The augite is generally green, but 

 occasionally brown, and like the felspar includes the rarer minerals. 

 The green augite often approximates to Uralite, and is sometimes 

 changed to a fibrous chloritic substance. Biotite is generally mixed 

 with augite and hornblende. Titanite only occurs when the rock is 

 massive. Augite syenites occur in the Vosges in dykes. A Uralite 

 syenite occurs in the Ural at Turgojak. 



Foyaite, Elaeolite Syenite. Where the continuation of the Sierra 

 Morena enters Portugal it forms the Sierra de Monchique, the north- 

 west of Algarve. This range consists of Devonian slates and sandstones, 

 through which rise the dome-shaped masses of crystalline rocks known 

 as the Foya, 2968 feet high, and the Picota, 2410 feet high. These 

 crystalline rocks cover an area of about 84 square miles. From Foya 

 this rock is named Foyaite ; it is a nepheline syenite, in which the 

 naked eye easily distinguishes orthoclase, the elaeolite variety of nephe- 

 line, and hornblende. Orthoclase is predominant ; elaeolite shows hex- 

 agonal outlines ; hornblende occurs in long, slender, greenish-black 

 prisms. The accessory minerals are brownish-yellow titanite, dark lamel- 

 lar biotite, magnetite, and occasionally pyrite. Under the microscope, 

 nosean and sodalite occur as accessories with triclinic felspar, muscovite, 

 haematite, and apatite. The orthoclase is nearly always crystallised 

 with layers of oligoclase. The crystals of elaeolite are usually ill- 

 defined. The hornblende and augite are in almost equal quantity, and 

 intergrown with each other. Both are green. The nosean and sodalite 



