
ALTERATION AND METAMORPHISM 217 
quartz and mica, and as metamorphism advances, schistosity becomes 
more and more pronounced—the foliation being developed along pre- 
existing planes of division, as bedding or cleavage. Such a rock thus 
gradually merges into mica-schist or andalusite-mica-schist, often 
containing cordierite. This schistose rock in its turn eventually may 
become transformed, in close proximity to the batholith, into an exceed- 
ingly hard compact hornfels, in which no trace of schistosity may be 
observed. 
In the aureole surrounding a batholith greywacké may undergo 
similar changes. Knots may be developed in them, and if the original 
rocks contained much felspathic matter, they may be transformed into 
rudely foliated or gneiss-like mica-quartz-rock, with cordierite in less or 
greater abundance. The metamorphism of sedimentary rocks being 
dependant on their chemical character it is obvious that the succession 
of changes witnessed in the neighbourhood of intrusive masses must 
vary with the varying nature of the surrounding rocks. Limestone, 
for example, is transformed into marble, through which new minerals 
are disseminated, such as tremolite, lime-garnet, idocrase, zoisite, and 
other lime-silicates. These new minerals doubtless represent the 
impurities (sand, clay) diffused through the original unaltered limestone. 
When they are very abundant the rock passes into calc-sclicate hornfels. 
Pure siliceous sandy rocks are changed into quartzites and hard 
jaspideous schists; but should the original unaltered rocks have con- 
tained argillaceous matter, this is sure to be represented by the 
development of new minerals, such as mica. The molecular rearrange- 
ment of rock-ingredients and the chemical recombinations which 
‘result in the production of “new” minerals is one of the most notable 
phenomena of thermal and regional metamorphism alike. Equally note- 
worthy is the appearance of schistosity, which so frequently accompanies 
extreme rock-change. In thermal metamorphism, however, this structure 
is usually met with only in the immediate neighbourhood of a batholith, 
and it is not always present—the rocks in contact with an eruptive mass 
often appearing as highly compact, fine-grained, or coarsely crystalline 
rocks (“hornfelses”) without any trace of foliation. 
Schists and even igneous rocks, when they are traversed by batholiths, 
become metamorphosed, but the changes induced are less striking, and 
consist chiefly of recrystallisation and structural modifications. Schists, 
for example, may become highly contorted and puckered as they approach 
a batholith. Igneous rocks, likewise, are affected by plutonic intrusions 
—rearrangements and recombinations of their ingredients taking place, 
changes which are usually accompanied by the development of new 
minerals. 
Not only are rocks of all kinds more or less metamorphosed by 
intrusive masses, but the igneous masses themselves are not infrequently 
affected by the rocks amongst which they have been intruded. Some 
remarkable examples have been cited by French geologists. In the 
Pyrenees, for instance, normal granite in contact with calcareous strata 
becomes hornblendic and passes into diorite, which may or may not 


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