739 
gneisses among the rocks of the Pilandsbergen. This great similari- 
ty of structure points to analogy in the genesis; the principal 
factors for the formation of contactrocks are abundance of pneuma- 
tolytie gases, and crystallization at a low temperature, whilst recrys- 
tallisation under pressure in firm rock, as takes place in crystalline 
schists, will lead to a similar structure, as a sudden and rapid erystal- 
lization does under pressure in an already viscous magma, circum- 
stances under which the normal laws ruling crystallization in a 
slowly cooling magina are no longer in force. 
The varieties of lujaurite rich in aegirine or arfvedsonite, in 
which the feldspars are only developed as small crystals, show e. g. 
a great outward resemblance to some amphibolites. Characteristic 
of both of these is a simultaneous crystallization through the entire 
magma, or a simultaneous recrystallization through the entire rock, 
the consequence of which is the simultaneous formation of many 
little crystals. It will, however, generally be possible to distinguish 
between them, because in the igneous rocks with the habitus of 
crystalline schists the minerals that had already crystallized as pheno- 
erists under the influence of affinity, chemical equilibrium and relative 
solubility of the components, before the conditions of rapid crystalli- 
zation set in, will continue to exist as such, whilst with the crystalline 
schists the entire preexisting mixture of minerals must adapt itself to 
ihe new circumstances and recrystallize at the same time, in conse- 
quence of which the formation of idiomorphic crystals is checked 
allthrough the rock, and this is also the case in the genesis of 
contactrocks. A greater molecular mobility however often still allows 
the development of idiomorphic crystals and of a succession of 
erystallizations in a cooling igneous magma. These differences can 
often distinctly be observed in the sieve-structures which are met 
with in both groups of rocks. 
Defining the erystalline schists as metamorphic rocks, the lujau- 
rites do not belong to this group; neither do many other rocks 
often regarded as crystalline schists. As such may be mentioned 
the rocks of numerous gneiss-areas situated outside large folded 
mountain-ranges which just as the above described nepheline-syenites 
show strongly varying types, whereas traces of dynamometa- 
morphism after the consolidation are entirely wanting. 
Finally the above mentioned facts and reasonings illustrate the 
importance of the agencies which are at work in the cases of 
piezoerystallization and piezocontactmetamorphism, with ‘regard to the 
genesis of rocks of the habitus of the crystalline schists. 
48 
Proceedings Royal Acad. Amsterdam. Vol. XV, 
