308 NEW ZEALAND NATURE-STUDY BOOK 
metamorphism is mainly due to the presence of heated 
masses of igneous rock. 
But foliated crystalline rocks occur over such vast areas 
that their metamorphism cannot be explained by the action 
of intrusive igneous masses. Indeed they are frequently 
unconhected with any igneous rocks whatever. Most of 
their rocks exhibit foliation or the arrangement referred to 
previously——moreover the crystals are arranged in definite 
planes so as to give a grain to the rocks. The opinion is 
that these rocks, originally sedimentary deposits, were 
carried to a considerable depth from the surface, where 
partly by the internal heat of the earth and the heat caused 
by friction between the particles as the masses were being 
compressed, (in the presence of water), and partly by the 
pressure and folding to which they were subjected, their 
entire characters were changed. Such masses exhibit 
regional metamorphism, well seen in parts of Otago where 
large areas of schistose or foliated rocks occur. 
Amongst the best known metamorphic rocks are Granite, 
Gneiss, Schist rocks, and Slate. 
Granite we have included amongst the igneous rocks, and 
have already described. Its origin is not always very 
clear. Sometimes it is undoubtedly an igneous rock, but 
its mode of occurrence at other times leaves little room for 
doubt that it is an altered sedimentary deposit—in other 
words a metamorphic rock. 
(FNEISS. 
This rock consists practically of the same constituents 
as Granite, from which it differs mainly in the foliated 
arrangement of the minerals. 
Note its crystalline structure and the banded appearance 
of the mass—the light coloured felspars and quartz 
and the dark folia of mica. Determine whether the 
