204 GEOGNOSY AND GEOLOGY. 
and polymygnite. There are also veins of trap, dumb veins, and ore veins 
with gold and platinum, brown iron-stone, and quartz. 
The weathering of feldspar in syenite proceeds much in the same manner 
as in granite. The hornblende resists the decomposition longest, and thus 
gives rise to a roughness of the stone. Even this substance, however, is 
forced to yield in time to chemical forces, and a dark ferruginous soil is the 
result, as favorable to vegetation as that from granite. 
Syenite most frequently occurs in feldspathiferous pseudo-strata of the 
bottom series, as in gneiss. It is also found in nests and veins in the 
transition slate. Syenite has been met with of more recent origin than the 
oolite and chalk. 
In its other relations syenite is very similar to granite, especially in the 
accompanying phenomena. The distribution is much more limited than 
that of granite. It is found principally in Sweden, Norway, Finland, 
Germany, France, on Mount Sinai, in Greenland, in South and North 
America, and in some other parts of the earth. 
C. Porphyry Rocks. 
Under this head belong porphyries of various kinds. The principal are : 
eurite-porphyry, claystone-porphyry, and silicious porphyry. They have 
generally parallelopipedal cleavage, although both columnar and globular 
forms occur. Quartz is often separated in a pure state; which, however, 
is not the case on the newer porphyries, from which, then, they are some- 
times distinguished as quartziferous porphyry. Porphyry rocks generally 
resist well the action of the atmosphere. The mountains present themselves 
sometimes as sharp combs, acute pyramids or cones; sometimes as high 
dome-shaped elevations (porphyry mountain near Kreuznach, pl. 53, fig. 4), 
with a greater or less quantity of loose rocks and stones about the base. 
The internal uniformity of porphyry masses is sometimes interrupted by 
nests or beds of kaolin or magnetic iron-stone. Veins are not of frequent 
occurrence. They are see ties ferriferous, manganiferous, plumbiferous, 
argentiferous, &c., with various gangues. 
Eurite-, clay-, and hornstone-porphyry, both in the bottom series and in 
the transition slate, occur in nests, veins, and beds between the strata; the 
formation most frequently consists of porphyry that is younger than the 
carboniferous, but older than the zechstein. The occurrence of porphyry 
younger than the variegated sandstone has not yet been satisfactorily 
indicated. 
A porphyritic breccia sometimes presents itself as a product of attrition, 
connected with the elevation of these abnormal masses. The contiguous 
rocks are shattered, and the pieces cemented together by an earthy mass, 
the result of the consequent grinding together of the rocks. 
The porphyries already adduced are ae in prominent positions on the 
Scandinavian peninsula, in Great Britain, Germany, France, the Altai 
Mountains, Mount Sinai, and in various parts of America. 
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