556 



REPORT OF NATIONAL MUSEUM, 1890. 



ampbibole or pyroxene group. A soda-lime feldspar is nearly always 

 present and frequently microcliue ; otber common accessories are apa- 

 tite, zircon, and tbe iron ores. 



Chemical composition. — In column I below is given tbe composition 

 of a bornbleude syenite from near Dresden, Saxony, and in II tbat of a 

 mica syenite (minette) from tbe Odenwald. 



Silica 



Alumina . . . 

 Ferric iron . 

 Ferrous iron 

 Magnesia... 



Lime 



Soda 



Potash 



Ignition 



Per cent. 



Per cent. 



60.02 



57. 37 



10. 6G 



13.84 



i 7.21 



( 2. 44 

 £ 3.44 



2.51 



6.05 



3.59 



5.53 



2.41 



1.53 



6.50 



4.47 



1.10 



3.17 



Structure. — Tbe structure of tbe syenites is wbolly analogous to tbat 

 of tbe granites and need not be furtber described bere. In process of 

 crystallization tbe apatite, zircon, and iron ores were tbe first to sepa- 

 rate out from tbe molten magma, and bence are found in more or less 

 perfect forms inclosed by tbe feldspars and later formed minerals. 

 These were followed in order by tbe mica, hornblende, or augite, and 

 lastly tbe feldspars, the soda-lime feldspars, when such occur, form- 

 ing subsequent to tbe orthoclase. 



Color. — The prevailing colors, as shown by the collection, are various 

 shades of gray, through pink to reddish. 



Classification and nomenclature. — According as one or the other of 

 the accessory minerals of the bisilicate group predominates we have 

 (1) hornblende syenite, (2) mica syenite, or minette, and augite syenite. 



Otber varietal names have from time to time been given by various 

 authors. The name Minette, first introduced into geological nomencla- 

 ture by Voltz in 1828 (Teall), is applied to a fine-grained mica ortho- 

 clase rock, occurring only in the form of dikes and further differing 

 from the typical syenites in having a porphyritic rather than granitic 

 structure. (See specimens 36290 to 36299 inclusive, from Baden, Sax- 

 ony, and various parts of Germany.) Vogesite (specimen 70025 from 



lar rocks consisting of feldspar and hornblende, either with or without quartz. 

 Later, when a more precise classification became necessary, the German geologists 

 reserved the name syenite to designate only the quartzless varieties of these rocks, 

 while the quartz-bearing varieties were referred to the hornhlendic granites. This 

 is the classification now followed by all the leading penologists and is therefore 

 adopted here. Much confusion has arisen from the fact that the French geologist 

 Roziere insisted upon designating the quartz-bearing rock as syenite, a practice 

 which has been followed to a considerable extent both in this country and England. 



