PETROGBAPHIC CHARACTERS. 11 
lowing questions: Why should one part oi a sedimentary formation have been 
altered into roofing slate and an adjacent part of the same into schist? Would con- 
tinued metamorphism transform a roofing slate into a schist? Were schists of sedi- 
mentary origin at one stage roofing slate? Why should metamorphism begin in some 
shales by the formation of slip cleavage leading to schist, and in others by slaty 
cleavage? 
As to the mineral constituents of slate, Sorby gives the average size of the scales of 
sericite" in the best Welsh slates a- one two-thousandths of an inch in hreadth by one 
six-thousandth in thickness. & Scattered about anion-' the meshes of the muscovite are 
minerals which extinguish irregularly. Chief among these is quartz, mostly in clasl ic 
grains, sometimes formed in situ or in lenses of chalcedony. This secondary quartz 
may attach itself to the quartz fragments or to the various other minerals to be named. 
Next in importance is a chloritic mineral, dichroic (light yellow, dark green), polar- 
izing from a Prussian orplum blue to a violet or olive when the section is transverse 
to the scale, but remaining dark between crossed nichols during a complete revolu- 
tion when the section is parallel to it. These scales are usually intergrown with 
lamellae of muscovite, sometimes of biotite, and lie — some transverse to the cleavage 
and parallel to the grain, others parallel to the cleavage, c Chlorite and quartz some- 
times make up minute lenses with their long axes parallel to the cleavage, or chlorite 
surrounded by sericite makes lenses with their long axes in the grain direction. 
These scales of chlorite sometimes contain needle-like crystals, probably rutile, cross- 
ing one another at angles of 60° and 120° (see p. 80). This chloritic mineral is 
regarded as of secondary origin. Rutile needles are generally, but not always, pres- 
ent, and in great abundance.' 7 This mineral occurs also in irregular masses up to 
0.05 mm. in diameter, consisting of a network of prisms forming angles of 00° and 
120° (sagenite twins). Thus at Bremo, Va., as stated on page 115, hematite occurs in 
very minute scales and dots and very abundantly in the reddish and purplish slates. e 
There are also cubes, lenses, and spherules of pyrite. The spherules abound in 
black slates. Carbonaceous matter and graphite abound in the black and dark-gray 
slates. Clastic grains of feldspar and of zircon are characteristic Hemimorphic 
prisms of tourmaline are not very unusual. 
Calcite and other carbonates, particularly one of lime, iron, and magnesia, are apt 
to be evenly disseminated in rhombs and plates. Rhodochrosite MnC< > :; ) probably 
also occurs (see p. 71). The following have also been identified : Ottrelite, staurolite, 
garnet, andalusite, sphene, anatase, biotite, hornblende, epidote, apatite, pyrrhotite, 
gypsum, magnetite, limonite, pyrophyllite, talc. 
Slates are often speckled with minute protuberances which under the microscope 
resolve themselves into the lenses just referred to — so-called "eyes" or "knots." 
Instead of consisting of chlorite and quartz, they may consist of an octahedron of 
magnetite partially surrounded by quartz and that entirely by chlorite./ This quartz 
is then regarded as a later infiltration into a cavity formed between the magnetite 
aSee Laspeyres, Sericit: Zeitschr. fur Kryst., vol. 4, 1880. p. iT-M. 
b Sorby, ibid. 
cSee Renard, Recherches sur la composition et la structure des phyllades ardennais: Bull. Mas. roy. 
d'hist. nat. de Belgique, vol. 3, p. 235, pi. 12; Zirkel, Lehrb., 1894, p. 298; and G. Rose, Uber die regel- 
maasige Verwachsung der verscfiiedenen Glimmerarten mit einander sowie mit Pennin und Eisen- 
glanz: Monatsber. K. Akad. zu Berlin, 1869. 
''On these see Zirkel. Ueber die mikroskopische Zusammensetzung von Thonschiefern und 
Dachsehiefern: PoggendorfE Annalen, vol. 144. 1891, p. 319; Van Werweke, N. Jahrb. Min. Geol., 1881, 
vol. 1, p. 178; Saner. N. Jahrb. Min. Geol.. 1881, vol. 1. p. 227-238; Cathrein, Ueber das Vorkommen 
mikroskopischer Zirkone u. Titan-Mineralien in den Gesteinen, Wurzburg, 1884; Kalkowsky, Ele- 
mente der Lithologie, 1886, p. 257. Sorby (loc.cit., p. 68) gives the diameter of the slate needles as Less 
than -oooo inch. 
e Renard gives the size of the granules of Fe.,0 3 as 0.020-0.005 mm.; op. cit., vol.. 3, p. 234. See also 
his PI. XII. fig. 2, of purple and green slate, in same volume. See also Gosselet (Etudes sur Torigine 
de l'ottrelite: Ann. Soc. Geol. du Nord, vol. 15, 1887-1888, Lille. 1888, pp. 188-189) who describes hematite 
as occurring in three forms in the reddish slates: (1) In irregular grains 0.01 to 0.02 mm. or less; (2) 
in scales with a bluish steel-like luster under a mixture of reflected and transmitted light; (3) in 
minute granules which are always red. a brick-red under reflected light. 
/SeeGeinitz. Der Phvllit von Rimogne in der Ardennen: Tschermaks Min. Mittheil. (new :■ 
vol. 3; also Renard, op. "cit., vol. 2, 1883, p. 133 et seq., and pi. 6. 
