Parr IL § vi] CRYSTALLINE ROCKS—SCHISTOSE. 121 
is not specially abundant, though it occurs in considerable mass in 
the Alps (Mont Blane, Monte Rosa, Carinthia, &c.), and is found also 
among the Apennine and Ural mountains. 
Chlorite-schist.—A scaly schistose aggregate of greenish 
chlorite-usually with quartz and often with felspar, talc, mica, or 
magnetite, the last-named mineral frequently appearing in beautifully 
perfect disseminated octohedra. Occurs with gneiss and other 
schists in evenly bedded masses. } 
Hornblende-schist.—A schistose mass of black or dark-green 
hornblende, but often interleaved with felspar, quartz, or mica. . 
When the schistose character disappears the mass becomes a horn- 
blende-rock (amphibolite). When the variety actinolite cecurs” 
instead of common hornblende it forms actinolite-schist. These 
hornblende rocks occur as bands associated with gneiss and other 
schistose formations. It was suggested by the late Mr. Jukes that 
_ they may possibly represent what were once beds of hornblendic or 
augitic lava and tuff which have been metamorphosed together with 
_ the strata among which they were intercalated. 
Clay-slate, argillaceous-schist (Argillite, Phyllite, Schiste 
ardoise, Thonschiefer, Thonglimmerschiefer), Under these names are 
included certain hard fissile argillaceous masses composed primarily 
of compact clay, with usually minute flakes of mica, fine granules of 
quartz, and frequently cubes and concretions of pyrites as well as 
veins of quartz and calcite. The fissile structure is specially 
characteristic. In some cases this structure is merely that of original 
deposit, as is proved by the alternation of fissile beds with bands of 
hardened sandstone or even conglonierate. Such are the argillaceous 
schists of the Scottish Highlands. But in certain regions where 
the rocks have been much compressed the fissile structure of the 
argillaceous bands is independent of stratification, and can be seen 
traversing it. Sorby has shown that this superinduced fissility or 
“cleavage” has resulted from an internal rearrangement of the 
particles in planes perpendicular to the direction in which the rocks 
have been compressed (See Book II. Section iv. § iii). In England 
the term “slate” or “clay-slate” has generally been applied solely 
to argillaceous rocks possessing this cleavage-structure. Those 
where the fissility is that of original sedimentation may be called 
“ argillaceous schists.” 
Microscopic examination shows that while some argillaceous rocks 
consist mainly of granular kaolin, many cleaved clay-slates contain 
a large proportion of a micaceous mineral in extremely minute 
flakes which in the best Welsh slates have an average size of =, 
of an inch in breadth, and ,2;,; of an inch in thickness, together 
with very fine black hairs which may be magnetite. Moreover, 
many clay-slates, though to outward appearance thoroughly non- 
erystalline and evidently of fragmental composition and sedimentary 

1 Sorby, Q. J. Geol. Soc. xxxvi. p. 68. See also a paper on the microscopic structure 
of Huronian clayslates by A. Wichman, op. cit. xxxy. p. 156. 
