GEOGNOSY AND GEOLOGY. 203 
of silex, and a proportion of water, remain behind, and form a white, fine- 
grained, unctuous mass, kaolin, which is an important ingredient in pottery. 
The soluble silicate of potassa furnishes the potassa so necessary to the 
plant, and is represented by the formula K3 Si®, while the aluminous 
silicate =Al? Sit +6H. All feldspathic rocks experience the same action, 
as weiss-stein, gneiss, syenite, some feldspathic porphyries, &c. The 
magnetic polarity of some granitic rocks is somewhat remarkable. The 
observation was first made at Ilsenstein in the Hartz, subsequently on the 
Schnarcherklippe, also in the Hartz. Granite occurs in many different 
periods of normal deposits. In Sweden it is older than the transition slate, 
which is shown from the fact that it had upheaved the gneiss, and become 
melted into it, and that then the transition slate rocks were deposited in 
horizontal nonconformable strata. The granite of Esterelle in Provence is 
older than the red sandstone. In Sweden, some granites are younger than 
the transition slates; they have broken through the old red sandstone and 
overlie it. There are granites in England which are newer than the 
carboniferous, but older than the new red sandstone. In the Alps, granites 
overlie the Jura, and, in the Pyrenees, have broken threugh the cretaceous. 
The granite is often traversed by other plutonic rocks, as by syenite, eurite, 
porphyry, and trap ; it often itself traverses syenite and various pyroxenes. 
The phenomena accompanying the presence of granite clearly testify to 
its plutonic origin. Masses in its vicinity are generally of greater density 
than those further removed ; grauwacke is changed into hornfels, clay slate 
into silicious slate, and sandstone into quartz rock. The relation borne by 
hornfels to granite is frequently very interesting. The former often 
constitutes a thin covering for the latter, inclosing it in an envelope. 
Again, it sometimes constitutes a cap to the granite. This is well seen on 
the Achtermanns heights in the Hartz. Not far from there, on the Rehberg 
cliff, the granite has passed in veins into hornfels. Sometimes single 
fragments of the granite are inclosed by hornfels, and the reverse ; and if 
pieces of limestone are present, they are changed into marble. Granite is 
most extensively distributed: it forms a constituent of almost all mountains 
throughout the entire earth. Exceptions to the general rule do, however, 
occur, as it is not found in the Sierra Nevada and the Jura chain. 
B. Syenite Rock. 
Syenite, although small in quantity in proportion to granite, yet stands in 
precise relation to it. It is frequently found in connexion with normal 
masses, and only rarely occurs isolated. When in considerable quantity, it 
is generally shattered, furrowed by deep valleys, whose sides are studded 
with rocks. The rock features of syenite are much like those of granite ; 
the cleavage also is similar, although less regular. 
Syenite is not unfrequently traversed by veins of newer syenite, of 
different petrographical character. In these veins, again, are sometimes 
found other veins of very interesting minerals, as eleeolite, beryl], pyrochlore, 
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