
Parr Il. § vi] FRAGMENTAL ROCKS—VOLCANIC. 161 
Fuller’s Earth (Terre 4 foulon, Walkerde).—A greenish or brownish 
earthy, soft, somewhat unctuous substance, with a shining streak, 
which does not become plastic with water, but crumbles down into mud, 
It is a hydrous aluminous silicate with some magnesia, iron-oxide and 
soda. The yellow fuller’s earth of Reigate contains silica 44, 
alumina 11, oxide of iron 10, magnesia 2, lime 5,soda 5... In England 
fuller’s earth occurs in beds among the Jurassic and Cretaceous forma- 
tions. In Saxony it is found as a result of the decomposition of 
diabase and gabbro. 
Wacke.—A dirty green to brownish-black earthy or compact, but 
tender and apparently homogeneous clay, which arises as the ultimate 
stage of the decomposition of basalt-rocks an situ. 
Till, Boulder-clay.—A stiff sandy and stony clay, varying in 
colour and composition, according to the character of the recks ef the 
district in which it lies. It is full of worn stones of all sizes, up to 
blocks weighing several tons, and often well smoothed and striated. 
It isa glacial deposit, and will be described among the formations 
of the Glacial Period. 
Mudstone.—A fine, usually more or less sandy, argiilaceous rock, 
having no fissile character, and of somewhat greater hardness than 
any form of clay. The term Clay-roek has been applied by some - 
writers to an indurated clay requiring to be ground and mixed with 
water before it acquires plasticity. 
Shale (Schiste, Schieferthon).—A general term to describe clay that 
has assumed a thinly stratified or fissile structure. Under this term 
are included laminated and somewhat hardened argillaceous recks 
which are capable of being split along the lines of deposit into thin 
leaves. They present almost endless varieties of texture and eom- 
position, passing on the one hand into clays, or, where much in- 
durated, into slates and argillaceous schists, on the other into flagstones 
and sandstones, or again, through calcareous gradations into limestone, 
or through ferruginous varieties into clay-ironstone, and through 
bituminous kinds into coal. Some of the altered kinds of clay-rocks 
have already been described. Flinty-slate or Lydian-stone and clay- 
slate are merely forms of clay that have undergone change from 
pressure or infiltrating solutions (see pp. 117, 121). 
3. Volcanic Fragmental Rocks—Tuffs. 
This section comprises all deposits which have resulted from the 
comminution of voleanic rocks. They thus include (1), those which 
consist of the fragmentary materials ejected from volcanic foci, or the 
true ashes and tuffs; and (2),some rocks derived from the superficial 
disintegration of already erupted and consolidated volcanic masses. 
Obviously the second series ought properly to be classed with the 
sandy or clayey rocks above described, since they have been formed in 
Y Ure’s Dict. Arts, &c. ii. p. 142, 
M 
