Parr Il. § vi] CRYSTALLINE ROCKS—SCHISTOSE. 128 
mineral.’ In Fruchtsehiefer these concretions are like grains of corn ; 
in Garbenschiefer, like caraway seeds ; in leckschiefer, like flecks or 
spots. Some of these rocks might be included with the mica-schists. 
Anthracitic-slate, Alum-slate, dark carbonaceous slate 
with much iron disulphide. Bands of this nature sometimes run 
through a clay-slate region. The carbonaceous material arises from 
the alteration of the remains of plants (fucoids) or animals (fre- 
quently graptolites). The marcasite so abundantly associated with 
these organisms decomposes on exposure, and the sulphuric acid 
produced, uniting with the alumina, potass, and other bases of the 
‘surrounding rocks, gives rise to an efflorescence of alum, or the 
decomposition produces sulphurous springs like those of Moffat. 
Mica-schist (Mica-slate).—A schistose aggregate of quartz and 
mica, the relative proportions of the two minerals varying widely 
even in the same mass of rock. Hach is arranged in lenticular wavy 
lamine. The quartz shows greater inconstancy in the number and 
thickness of its folia. Frequently a layer of this mineral swells out 
to a thickness of an inch or more, and, dwindling rapidly down to a 
mere thread, disappears. ‘The quartz often retains a granular 
character like that of quartz-rock, no doubt indicative of its 
original sedimentary origin. The mica lies in thin plates, some- 
times so dovetailed into each other as to form long continuous 
irregular crumpled folia, separating the quartz layers, and often in 
the form of thin spangles and membranes running in the quartz. 
(Figs. 18 and 19). As the rock splits open along its micaceous folia, 
the quartz is not readily seen save in a cross fracture. 
Muscovite is the usual mica in typical mica-schist; but it is 
sometimes replaced by biotite. In many lustrous schists which are 
now found to have a wide extent, the silvery foliated mineral is 
ascertained to be a hydrous mica (margarodite, damourite, &c.), and 
not tale, as was once supposed. ‘These, as already stated, have been 
named hydro-mica-schists. Among the accessory minerals, garnet, 
schorl, felspar, hornblende, kyanite, staurolite, chlorite, and tale may 
be mentioned. Mica-schist readily passes into other members of 
the schistose family. By addition of felspar it merges into gneiss. 
By loss of quartz and increase of chlorite it passes into chlorite- 
schist, and by other gradations into quartz-rock, &c. 
Mr. Sorby has pointed out that thin slices of true mica-schist 
when examined under the microscope show traces of the original 
grains of quartz-sand and other sedimentary particles of which the 
rock at first consisted. He has also found indications of current- 
bedding or ripple-drift, such as may be seen in many fine sedimentary 
deposits, and he concludes that mica-schist is merely a crystalline 
metamorphosed sedimentary rock.? Besides the original quartz- 
1 A. von Lasaulx, Neues Jahrb. fiir Min. (1872), p. 840. K. A. Lossen, Z. 
Deutsch. Geol. Ges. (1872), p. 757. 
2 Q. J. Geol. Soc. (1863), p. 401, and his recent address in vol. xxxvi. (1880), p. 85. 
