138 GEOLOGY. 
ashes, is known as trap-tuff, or tufa, a word that comes from Italy, 
the seat of so much volcanic action. 
Structure of Rocks.—On examining the rocks forming the crust of 
the earth, we find that they may be divided into two great classes—the 
stratified, or those deposited in strata or layers ; and the unstratified, or 
those not so formed. Sandstone and slate are stratified rocks; granite 
and trap are unstratified. 
1. Stratified Rocks —Any thin deposit of rock is called a layer, from its 
having been laid down under water ; a band, from its being like a thin 
band; a bed or a stratum, when of greater thickness, from Latin sterno, to 
spread ; and a seam, when of a peculiar character as compared with the 
rocks near it, as a seam of coal. Stratum, with the plural strata, is the 
general term for any layer of rock, and hence all rocks in layers are 
said to be stratified. Rocks that split up into very thin layers, a great 
number being included in the thickness of an inch, are said to be 
laminated. When a rock is composed of rounded pebbles or boulders 
embedded in other matter, it is called a conglomerate, and sometimes, 
from its appearance, pudding-stone or plum-pudding-stone. 
2. Unstratified Rocks.—Unstratified rocks assume various forms, accord- 
ing as they have been shot up amongst the stratified rocks ; for, as we 
shall afterwards see, they have be erupted from volcanoes. Very 
often, like most volcanic substances, they are porous or cellular, like 
pumice-stone ; frequently they stand together like gigantic columns, when 
they are said to be columnar, like basalt ; and often they are found in 
large globular or spherical masses, like bombs or cannon-balls. 
Disposition of Rocks.—1. Stratified Rocks—When rocks lie parallel © 
to the horizon, they are termed flat or horizontal, as A, in the following 
section ; when at an angle to it, they are said to be inclined or dipping, 
as B; when one end has been thrown up by some other mass, they are 
said to be tilted wp, as R; when so much inclined as to be straight up 
and down, they are said to be perpendicular, or to stand on edge. When 
inclined rocks come to the surface, they are said to crop out, and the 
exposed edge is therefore termed the owtcrop, as M; the angle at which’ 
they are inclined is called the dip of the rocks, and is measured by the 
number of degrees from the horizontal in any direction, as 60° S.; and 
the line of the outcrop along the surface is termed the strike or line of 
strike, because it strikes or runs across the country. When the strata are 
not straight, they are said to be bent or curved; and when greatly bent, 
twisted or contorted, as P. When all the strata in a series lie at the same 
angle, they are called conformable; when at different angles, wnconformable, 
as at A. Sometimes certain strata seem to have slipped or to have 
moved up, so that rocks that should be opposite to one another are not 
so. The portion that has slipped is naturally termed a slip, as C; 
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