Ak ae eee ae 
ae ‘' 2 
m+ 
4 
be 
‘ - 
Parr II. § vi] CRYSTALLINE ROCKS—MASSIVE. — 129 
structure from the development of micaceous scales, and which 
contains porphyritically scattered crystals of felspar and quartz. 
The felspar is either orthoclase or albite, and may be obtained in 
tolerably perfect crystals. The quartz occasionally presents doubly 
terminated pyramids. The micaceous mineral may be paragonite 
or sericite. Porphyroid occurs among the schistose rocks of 
Saxony,! in the palzeozoic area of the Ardennes,’ as well as in West- 
phalia and other parts of Europe. 
Before passing from the schistose series of rocks, the student will 
observe that the disappearance of the schistose structure produces a 
erystalline amorphous compound. In gneiss, for example, the same 
minerals which form granite have crystallized in a foliated manner. 
Any process, such as irregular internal motion of the mass that 
could change the schistose structure of gneiss into the massive 
structure of granite, would give rise to a rock which, whatever its 
previous history might have been, might not be distinguishable from 
granite. On the other hand, any internal rearrangement which 
could produce a foliated structure within a mass of granite, would 
present a rock that would deserve the name of gneiss. That such 
internal transformations have taken place among the crystal- 
line schists and some granites and other eruptive rocks can hardly 
be doubted. And thus, at the one end of the schistose series, we 
find rocks in which an original sedimentary character remains un- 
mistakable; while at the other, after many intermediate stages of 
progressively augmenting crystallization, we encounter thoroughly 
crystalline amorphous masses like granite and syenite, which should 
be placed among the massive rocks. This arrangement no doubt 
correctly represents what has been a real cycle of alteration among 
tocks. Sedimentary deposits have been gradually changed and 
erystallized. These metamorphosed products, by upheaval and 
exposure at the surface, have again been reduced to sediment, 
perhaps once more to pass through the same succession of altera- 
tions and to become yet again crystalline. 
3. Massive Rocks. 
This important sub-division is nearly coincident with what is 
embraced by the old and useful terms Igneous or Eruptive Rocks. 
Almost the whole of its members have been produced from within 
the crust of the earth, in a molten or at least in a pasty 
condition. Nearly all consist of two or more minerals. Considered 
from a chemical point of view, they may be described as mixtures 
in different proportions of silicates of alumina, magnesia, lime, 
potash, and soda, usually with magnetic iron and phospate of lime. 
In one series the silicic acid has not been more than enough to 
combine with the different bases; in another it occurs in excess as 
free quartz. Taking this feature as a basis of arrangement, some 
1 Rothpletz, Geol. Survey Saxony. Explanation of Section Rochlitz. 
2 De la Vallée Poussin and Renard, Mem. Couwronnées Acad. Roy. Belg. 1876, p. 85. 
é K 
