558 
ELEMENTS OF GEOLOGY. 
compound of feldspar and hornblende, and regarded quartz 
as merely one of its occasional minerals. 
Miascite is one of the varieties of syenite most frequently 
spoken of; it is composed chiefly of orthoclase and nepheline, 
with hornblende and quartz as occasional accessary miner¬ 
als. It derives its name from Miask, in the Ural Mountains, 
where it was first discovered by Gustavus Rose. Zircon- 
syenite is another variety closely allied to Miascite, but con¬ 
taining crvstals of Zircon. 
Connection of the Granites and Syenites with the Volcanic 
Eocks.^ —The minerals which constitute alike the plutonic and 
volcanic rocks consist, almost exclusively, of seven elements, 
namely, silica, alumina, magnesia, lime, soda, potash, and iron 
(see Table, p. 499) ; and these may sometimes exist in about 
the same proportions in a porous lava, a compact trap, and a 
crystalline granite. The same lava, for example, may be 
glassy, or scoriaceous, or stony, or porphyritic, according to 
the more or less rapid rate at which it cools. 
It would be easy to multiply examples and authorities to 
prove the gradation of the plutonic into the trap rocks. On 
the western side of the fiord of Christiania, in l^orway, there 
is a large district of trap, chiefly greenstone-porphyry and 
syenitic-greenstone, resting on fossiliferous strata. To this, 
on its southern limit, succeeds a region equally extensive of 
syenite, the passage from the trappean to the crystalline plu¬ 
tonic rock being so gradual that it is impossible to draw a 
line of demarkation between them. 
“ The ordinary granite of Aberdeenshire,” says Dr. Mac- 
Culloch, ‘‘ is the usual ternary compound of quartz, feldspar, 
and mica; though sometimes hornblende is substituted for 
the mica. But in many places a variety occurs which is 
composed simply of feldspar and hornblende; and in exam¬ 
ining more minutely this duplicate compound, it is observed 
in some places to assume a fine grain, and at length to be¬ 
come undistinguishable from the greenstones of the trap 
family. It also passes in the same uninterrupted manner 
into a basalt, and at length into a soft claystone, with a 
schistose tendency on exposure, in no respect differing from 
those of the trap islands of the western coast.” The same 
author mentions, that in Shetland a granite composed of horn¬ 
blende, mica, feldspar, and quartz graduates in an equally 
perfect manner into basalt.* In Hungary there are varieties 
of trachyte, which, geologically speaking, are of modern ori¬ 
gin, in which crystals, not only of mica, but of quartz, are 
common, together with feldspar and hornblende. It is easy 
* Syst. of Geol., vol. i., pp. 157 and 158, 
