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154 . GEOLOGY. 
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I.—Metamorphic System. 
Description. Immediately above the granite, and resting directly upon 
it, lie the most ancient sedimentary rocks in the long series of formations 
before the human era. These rocks do not exist now as they were laid 
down. The whole granite surface seems to have retained much of its 
heat, at least so much as to have changed the:character of the mud at the ~ 
bottom of the seas, and thus formed the hard crystalline rocks of this 
earliest sedimentary system. It is from being thus changed or metamor- 
phosed by heat that they have received the name of the Metamorphic System.+ 
They consist of various strata, of which the chief are Gneiss, Quartz-rock, 
Mica-schist, and Clay-slate. 
Gneiss,? so named from its thin layers, is a hard crystalline rock, in 
extremely thin bands, often twisted in a remarkable manner. It consists 
mainly of the same minerals as granite, from which it was formed, but 

Gneiss. Mica-schist. 
: 2 
Fig. 83. 
Granite. 
these are broken and confused. Quartz-rock, as its name implies, is 
composed of fine grains of quartz. Mlica-schist obtains its name from the 
particles of mica that form its chief ingredient. ‘These three kinds of rocks 
compose one great group, and, though frequently intermingled, occur in 
the above order. They are extensively developed in Britain, especially in 
the Highlands of Scotland, where they form the mass of the mountain 
ranges, and enter into the grand and beautiful scenery of that picturesque 
region. With them also occurs limestone, which, having been crystallised 
by heat, forms beautiful marbles, quarried for various purposes. 
Clay-slate is that most useful rock, which, split up into thin layers, forms 
the familiar blue slates for our roofs, and the slate and slate-pencil used 
every day in school. There is something very interesting in the thought, 
that the slates you may now see before you, or may be even now using, 
and the pencil in your hands, are obtained from the earliest of all our 
tock-formations, and are formed of the fine sand washed from the primi- 
tive granite, but lately cooled down! 
Organic Remains.—The Metamorphic rocks, from being the earliest of 
our rock-systems, are known also as the Primary rocks, or those first 
1 From Greek meta, change, and morphé, form. 2 From Anglo-Saxon gnidan, to rub. 
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