THE ORIGIN AND DESCENT OF ROCKS. 415 



A'ery few granites conform strictly to the type. They vary by 

 the addition and substitution of other minerals, and these sometimes 

 become as prominent as the type minerals. The soda-hme feldspars 

 sometimes take the place of the orthoclase, or accompany it; horn- 

 blende and other minerals take the place of the biotite, or occur T\ith 

 it ; and so on. ^ henever one of these replacing or accessory minerals 

 is notable in quantity, its name is often prefixed, as hornblende-granite, 

 ohgoclase-granite, zircon-granite, etc. In this way the rock grades 

 almost insensibly into the syenites, diorites, etc. Variations also arise 

 from the absence of one of the three leading minerals. If mica is absent, 

 the rock is termed Rnaplite (quartz and feldspar). If the feldspar is 

 absent, it is called a greisen (quartz and mica). If quartz is absent, 

 it is termed a minette (feldspar and mica). These varietal terms are 

 neither universally nor always consistently used, and it is to be hoped 

 they ^ill be replaced by the systematic nomenclature recently proposed 

 and outhned later (p. 451). 



The granites were formed from a magma rich in sihca, alumina, 

 potash, and soda, but generally poor in lime, iron, and magnesia. Inci- 

 dentally other substances were present. The alumina, potash, and other 

 bases miited T^ith so much of the silica as was required to form the 

 feldspars and micas, and the remaining sihca crystalhzed into quartz. 



Granite is normallv a massive rock without fohation or banding;. 

 If it takes on these characters, it becomes a gneiss, and passes into the 

 foliated or schistose class of rocks, to be discussed later. The texture 

 of grapliic granite ('see pegmatite) is notably peculiar, due to the simul- 

 taneous crystallization of the cpartz and feldspar (Fig. 345). 



The syenites. — ^lien the mica of a granite is replaced by horn- 

 blende, the rock is now commonly known as a Jiornblende-granite, but it 

 was formerly called syenite, because found at Syene on the Nile. The 

 term syenite is now apphed to a rock consisting essentially of feldspar 

 and hornblende or mica, but there is a complete gradation from the 

 granites to the syenites. The magma of the syenites was richer in iron 

 and magnesiimi than the typical granitic magma. The syenites also grade 

 into other classes, as do the granites, and are named by similar prefixes, 

 as augite-syenite, etc., and some of these A^arieties have special names. 

 The syenites are red or gray, according to the color of the feldspar, and 

 are usually darker than the granites. The texture of syenite is Hke 



