
fe eee 
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Parr IT. § vi] CRYSTALLINE ROCKS—SCHISTOSE. 119 
folia, which becomes sometimes so fine as to be discernible only under 
the microscope! (Fig. 19), but is often present conspicuously in hand- 
specimens (Fig. 18), and can be traced in increasing dimensions till it 
connects itself with gigantic curvatures of the strata which embrace 
whole mountains in their sweep. These characters are sufficient to 
indicate a great difference between schistose rocks and ordinary 
stratified formations, in which the strata lie in continuous flat, parallel, 
and more or less easily separable layers. 

Fic. 18.—Vimw oF A Hanp Specimen oF Contrortep Mica scmist, 
TWO-THIRDS NATURAL SIZE. 
A rock possessing this crystalline arrangement into separate folia 
is termed a “schist.” This word, though employed as a general 
designation to describe the structure of all truly foliated rocks, is also 
made use of as a suffix to the names of the minerals of which some of 
the foliated rocks largely consist. Thus we have “ mica-schist,” 
“ chlorite schist,” “hornblende-schist.” If the mass loses its fissile 
tendency owing to the felting together of the component mineral into 
1 On the microscopic structure of the crystalline schists see Zirkel, Microscopical 
Petrography (vol vi. of King’s Exploration of 40th Parallel) 1876, p. 14. Allport, Q. J. 
Geol. Soe. xxxii. p. 407. Sorby, op. cit. xxxvi. p. 81. 
