THE SYENITES 69 



(1) THE SYENITES 



The name Syenite, from Syene, a town of Egypt. The word 

 was first used by Pliny to designate the coarse red granite from 

 quarries at Syene, used by the Egyptians in their obelisks 

 and pyramids. Afterwards (in 1787) "Werner introduced the 

 word into geological nomenclature to designate a class of gran- 

 ular rocks consisting of feldspar and hornblende, either with or 

 without quartz. Later, when a more precise classification be- 

 came necessary, the German geologists reserved the name syenite 

 to designate only the quartzless varieties, while the quartz- 

 bearing varieties were referred to the hornblendic granites. 

 This is the classification now followed by the leading petrologists 

 and is therefore adopted here. Much confusion has arisen from 

 the fact that the French geologist Eoziere insisted upon desig- 

 nating the quartz-bearing rock as syenite, a practice which has 

 been followed to a considerable extent both in this country and 

 England. 



Mineral Composition. — The syenites differ from the granites 

 only in the absence of the mineral quartz, consisting essentially 

 of orthoclase feldspar in company with biotite, or one or more 

 minerals of the amphibole or pyroxene group. A soda-lime 

 feldspar is nearly always present and frequently microcline; 

 other common ac3essories are apatite, zircon, and the iron ores: 

 more rarely sodalite. 



Chemical Composition. — In column I on p. 70 is given the 

 composition of a hornblende syenite from near Dresden, Saxony, 

 in II that of a mica syenite (minette) from Odenwald, in III 

 that of an augite-sodalite syenite from Montana, and in IV that 

 of an augite syenite from Franklin Co., New York. 



Structxire. — The structure of the syenites is wholly analo- 

 gous to that of the granites, and need not be further described 

 here. 



Color. — The prevailing colors 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 predomi- 

 nates we have (1) hornblende syenite, (2) mica syenite, or min- 

 ette, and (3) augite syenite. 



Other varietal names have from time to time been given 

 by various authors. The name minette, first introduced into 



