I 
172 IDDINGS. 
phyritical crystals are sanidine and plagioclase, with a little 
augite, biotite, and magnetite. Its occurrence as a contem¬ 
poraneous flow in the midst of great accumulations of basalt 
and andesite, as well as its chemical composition, shows that 
it may be considered as a product of extreme differentiation. 
Its chemical analysis has been placed in Table III, and is 
number 20. In silica percentage it corresponds to some 
dacites, but it is lower in lime and magnesia and much higher 
in potash. It forms a connecting link between trachyte and 
rhyolite. 
“ Ganggesteine” in other parts of the world .—Let us con¬ 
sider briefly the general character of the rocks which Rosen- 
busch has grouped together as “ ganggesteine.” They are 
classed under three sub-groups: Granitic, granite-porphy- 
ritic, and lamprophyric. 
The first includes aplite and pegmatite. 
The second embraces granite-porphyry, syenite-porphyry, 
eleolite-syenite-porphyry, and diorite-porphyrite. 
The third includes the less common kinds of rock called 
lamprophyres, whose chief characteristic is the preponder- 
ence of phenocrysts of the ferromagnesian minerals and the 
scarcity or absence of those of feldspar and quartz. Accord¬ 
ing to the feldspathic component of the groundmass, they are 
classed as syenitic lamprophyres and dioritic lamprophyres. 
To the first belong minette and vogesite, and to the second 
kersantite and camptonite. 
These rocks have been called “ ganggesteine ” because they 
are said to attain their most typical development in dikes, 
though they sometimes appear as facies of large granular 
masses. They are said not to occur on the surface of the 
earth as extrusive bodies. In exactly what sense this is to 
be taken is not quite clear. It is, of course, understood that 
in the case of the highly crystalline kinds there are no ex¬ 
trusive equivalents that have the same crystalline structure; 
but their exact chemical equivalents are of frequent occur¬ 
rence, although Rosenbusch states that he finds the extru¬ 
sive equivalents of the granular rocks differ from them to 
