
46 STRUCTURAL AND FIELD GEOLOGY 
molten matter, if it were poured out at the surface, would 
solidify as an obsidian or a rhyolite; if it cooled at a very 
considerable depth it would consolidate into granite; while if 
it were injected in the form of sills and dykes at a less 
depth, some portions might become microgranite or quartz- 
porphyry, and others, which had cooled more rapidly, pitch- 
stone. 
Syenite, usually reddish but not infrequently grey, is a 
holocrystalline granitoid aggregate of orthoclase and a ferro- 
magnesian mineral, which may be hornblende, augite, or 
mica. Syenite is differentiated from granite by the absence 
of quartz. Nevertheless, under the microscope a little quartz 
can often be seen straggling among the other ingredients. 
The accessory minerals include plagioclase, which is rarely 
quite wanting, apatite, zircon, sphene, ilmenite, magnetite. 
Normal Syenite consists essentially of orthoclase and hornblende. 
Augite- or Pyroxene-syenite contains orthoclase and plagioclase, augite 
(sometimes diallage), hypersthene, biotite, and a little quartz. When the 
chief ferromagnesian mineral is biotite, we have the variety known as 
Mica-syenite. Lleolite-syentte is a compound of alkali felspar, elzeolite, 
and one or more ferromagnesian minerals (pyroxene, amphibole, mica). 
The rock is noted for the variety of its accessory ingredients, amongst 
which are plagioclase, sphene, apatite, zircon, fluor-spar, sodalite, and 
others. 
The syenites are not so widely distributed as the granites. The type- 
rock is that of the Plauenscher-grund, Dresden. Many varieties of 
syenite occur in S. Norway, and have received special names (Laurvzkite, 
Nordmarkite, etc.). Kentallenite is the name given to a basic syenite 
occurring in Argyllshire. 
Orthoclase-porphyry (Orthophyre) is a grey, brown, or reddish rock, 
the groundmass of which consists essentially of microcrystalline ortho- 
clase. Scattered through this are phenocrysts of orthoclase. Plagioclase 
is sometimes present, and needles of hornblende, scales of biotite, or 
granules of augite, may often be observed ; a little quartz, too, occasionally 
appears. ‘The most conspicuous ingredient, however, is the orthociase. 
When ferromagnesian minerals, such as biotite, are plentifully present, 
this rock passes over into Minette or Syendtic Mica-trap. ‘This rock, when 
fresh, is dark grey to black, but owing to weathering it is often brown. 
The texture is medium to fine-grained or compact. The microscope 
shows, however, that it is holocrystalline. 
Here also may be included Lostonzte, a light yellowish or grey rock, 
with a fine-grained to compact groundmass, composed chiefly of small 
lath-like crystals of felspar (see Plate IX. 3). Dark ferromagnesian 
minerals are very sparingly present. The rock occurs in dykes, and is 

