222 DISTRIBUTION OF SYENITE. 



inclusions contain small cubic crystals. The orthoclase twins gene- 

 rally follow the Carlsbad law ; they are sometimes white, but commonly 

 red. In many Scandinavian syenites and in the Thuringerwald, the 

 mineral has a blue lustre. Plagioclase, when present, resembles that 

 of granite. In augite-syenite, plagioclase is better preserved than 

 orthoclase. 



Hornblende is usually green ; when it is brown, as at Laurvig ill 

 Norway, the brown colour is deep as in trachytes and andesites. It 

 commonly occurs in lamellar or columnar crystals, and encloses mag- 

 netite, apatite, brown mica, and titanite. 



The biotite may be green or brown, or both colours occur in the 

 same film. It is associated with the hornblende. The crystals or 

 irregular grains of augite are usually green, but when the rock is very 

 rich in plagioclase, the colour is yellowish brown. Quartz occurs 

 occasionally, and is probably an accessory mineral due to alteration of 

 the orthoclase. 



The predominant mineral composition indicates the division of 

 syenite into three types, which may be termed hornblende-syenite, 

 mica-syenite, and augite-syenite. 



Geographical Distribution of European Syenites. Among the 

 European localities for syenite are Plauen, near Dresden, many places 

 on the southern slope of the Thuringerwald, in the Odenwald, Meis- 

 sen, in Saxony ; in Moravia it extends 30 miles from south of Kienitz, 

 through Brunn, to north of Boskowitz ; in the mountains of Lower 

 Silesia, a large mass of syenite extends from Glatz to Ullersdorf. A 

 rock of syenitic character, classed by Zirkel as a syenite-granite-por- 

 phyry, stretches from north to south in the east of the Banat from 

 Kudernatch to Moldawa. A somewhat similar rock occurs in the 

 Bihargebirge in south-east Hungary, penetrating Neocomian rocks. 

 In the Vosges, massive syenite appears between Windstein and Ballow, 

 north of Geromaguy. In the Tyrol it forms the centre of the eruptive 

 mass at Predazzo, and the great mountain mass of Monzoni, charac- 

 terised by red orthoclase, white oligoclase, with films of hornblende 

 and brown mica. At Monzoni, the Triassic limestone is converted for 

 a thickness of 100 feet into crystalline limestone, with the develop- 

 ment of many accessory minerals. In the South of Norway, syenite 

 is seen around Christiania, penetrating slates and limestones, and in 

 Finland it occurs near Viborg. 



North- American Syenite. There is only one exposure of syenite 

 in the region of the 4oth parallel survey. It forms the Cluro Hills 

 in the Cortez Eange, Nevada, and consists of flesh-red orthoclase and 

 greenish hornblende. Under the microscope, indications of plagioclase 

 are detected, and there are microscopic grains of quartz, with fluid 

 inclusions. 



Hornblende Syenite is generally large-grained, and formed of 

 orthoclase and amphibole, with titanite as an accessory. The type is 

 not so common as is usually supposed, but good examples are seen at 

 Plauen and Leuben in Saxony, and Biella in Piedmont. When the 

 rock is decomposed, calcite and epidote often appear as decomposition 



