SLATE. 
81 
more earthy in its texture. It forms beds of various 
degrees of thickness, and sometimes lofty and ser- 
rated mountains, which are covered with verdure 
on their declivities. Slate rocks vary much in qual- 
ity even in the same mountain ; some being hard 
and flinty, from containing a quantity of silicious 
earth ; others being soft and shelly, composed chief- 
ly of alumine. Indeed, flinty slate differs from com- 
mon slate only in containing a larger quantity of 
silex ; and when it loses the slaty character, it is 
called hornstone ; and if it contain crystals of feld- 
spar, it is called hornstone porphyry. Whetstones 
and hones are made of talcy slate containing silex ; 
and their excellence is proportioned to the fineness 
of the silicious particles and their uniform diffusion. 
Slate is universally regarded by geologists as a 
sedimentary rock, formed by the deposition of mi- 
nute particles of the primary rocks, in the state of 
mud, which has subsequently been consolidated by 
heat and pressure. In no other way can we account 
for the impression of animal and vegetable remains 
which are often found low down in the rock. 
Although slate splits into thin laminse, yet there 
are also contained in 
Fig. 28. it certain joints or 
fissures, traversing it 
in straight, well-de- 
termined lines, caus- 
ing it to cleave in a 
direction transverse 
to that of the beds, 
making with the la- 
minae an angle usual- 
ly of from forty to 
sixty degrees, as seen in figure 28. This affords to 
the quarrymau great aid in extracting blocks of a 
symmetrical shape. The same vertical joints or 
cleavings are met with also in limestone, and even 
in granite. If we make a paste of clay and starch, 
