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2 
122 ‘GEOGNOSY. - °° “fBoama 4 
origin, yet contain, sometimes in remarkable abundance, microscopic - 
microliths and crystals of different minerals. ‘These minute bodies 
consist of yellowish-brown needles possibly of hornblende, greenish 
or yellowish flakes of mica, also scales of calcite. They are generally 
placed with their long axes parallel with the lines of fissility. Small 
granules of quartz containing fluid-cavities, may possibly be of clastic 
derivation, but they show on their surfaces a distinct blending with 
the substance of the surrounding rock. M. Renard has found that 
the Belgian whet-slate is full of minute crystals of garnet.2 Yet the © 
original truly sedimentary origin of clay-slate is imdicated by its 
abundant clastic granules and flakes, by the traces of stratification, 
false-bedding, ripple-mark, &c., and by the occurrence of included 
organic remains. Some microscopic crystals may possibly have been 
originally formed among the muddy sediment on the sea-floor. But 
more probably they have been subsequently developed within the 
rock, and represent incipient stages of the process which has ended 
in the production of mica-schist and gneiss.? The development of 
crystals of chiastolite and other minerals in clay slate is frequently 
to be observed round bosses of granite as one of the phases of contact 
metamorphism. , 
A number of varieties of clay-slate are recognised. Roofing- 
slate (Dachschiefer) includes the finest, most compact, homo- 
geneous and durable kinds, suitable for roofing houses or the 
manufacture of tables, chimney-pieces, writing-slates, &c.; it occurs 
in the Silurian and Devonian formations of Central and Western 
_ Europe. Whet-slate, novaculite, hone-stone, an exceedingly 
\ hard fine grained siliceous rock, some varieties of which derive their 
economic value from the presence of microscopic crystals. Chiasto- 
lite-slate (schiste maclé), a clay-slate in which crystals of chiasto- 
lite have been developed, even sometimes side by side with still 
distinctly preserved graptolites or other organic remains ;* occurs 
at Skiddaw, also in Brittany, the Pyrenees, Saxony, Norway, 
Massachusetts, &c. Staurolite-slate, a micaceous clay-slate 
with crystals of staurolite; occurs in the Pyrenees. Ottrelite- 
slate a clay-slate marked by minute six-sided greyish or blackish 
green lamella of ottrelite ; occurs in the Ardennes (where it is said 
to contain remains of trilobites), also in Bavaria and New England. 
Dipyre-slate is full of small crystals of dipyre. German petro- 
graphers have distinguished by name some other varieties charac- 
terised by different kinds of concretions, but to which no special 
designations have been given in English. Knotenschiefer contains 
little knots or concretions of a dark-green or brown fine granular, 
faintly glimmering substance, of a taleose or micaceous nature, 
imbedded in a finely laminated matrix of a tale-like or mica-like 
' Zirkel, Mik. Beschaf’. p. 490. 
* Acad, Roy. Belgique, xli. (1877). 
® Sorby, loc. cit. See Book IV. Part viii. 
* A good illustration of this association is figured by Kjerulf in his Geologie de 
vee 72 ; 8 
Siidlichen und Mittleren Norwegen, Plate xiv. fig. 246, ie y 

