34 



METAMORPHIC ROCKS. 



Thus there are here (fig. 9) three planes of resistance, A, B, c, marked 

 by layers of crystals parallel to each of them, and the result is a 

 confused appearance, in which no single layer can be detected pre- 

 cisely like the aspect of granite. And although owing to the action 

 of resultant forces the particles might not be arranged exactly in the 

 directions here shown, the result would be the same in producing a 

 confused arrangement. Metamorphic granites, according to Professor 

 Haughton, may be seen in Donegal, some parts of Brittany, the Pyre- 

 nees, and many other localities. 



If now we suppose the rocks forming one of the sides A, B, c, to 

 have fractured, and to have opened in a fissure before the granite was 

 consolidated and cooled, then the pasty granitic rock would penetrate 



into the fissure and form an intru- 

 sive granite vein. Such veins may 

 be seen at Cape Wrath, and in 

 many parts of Scotland, in Corn- 

 wall, and wherever granite occurs. 

 Granite usually presents, where ex- 

 posed, more or less rounded out- 

 lines in the scenery. 



We are now able to advance 

 one step farther and appreciate the 

 close relation between granites and 

 lavas. The central cores of many 

 old volcanoes of the Auvergne in 

 France, and of the Hebrides, are 

 found to be granite ; and when this 

 rock cools more rapidly, as at the 

 earth's surface under the pressure of the atmosphere, the minerals no 

 longer form separately, but constitute rock, consisting more or less 

 obviously of a felspathic matrix in which crystals may occur. W r hen 

 poured out in a lava stream these rocks are called felstones, and when 

 they assume a looser texture become scorise or ashes. If now we 

 suppose the rocks over a central granite mass to become fractured 

 through their thickness so as to allow water to penetrate down to the 

 heated mass, and to form a funnel or vent out of which the heated 

 materials may escape, it is obvious that the central crystalline rocks 

 will throw out lavas and ashes which may build up a volcano. Thus 

 it follows that clay, slate, gneiss, granite, felstone, rhyolite, may all 

 exist simultaneously as different conditions of the same rock, which 

 have been produced in sequence to each other by the pressure which 

 also brings mountains into existence, and changes the outlines of 

 land and water. This ideal section (fig. n) will illustrate the rela- 

 tions of the several kinds of rocks to each other, and show the order 

 which the several classes of rocks may succeed each other on 



Fig. 10. 



in 



the flanks of a mountain range. Formerly all the igneous rocks 

 were classed into two groups first, volcanic, or eruptive, which had 

 burst out from beneath the earth's surface ; and secondly, plutonic 

 or crystalline rocks which cooled at great depths, and under great 





