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POPULAR SCIENCE REVIEW. 
have been respectively applied ; the first and third of these,, 
stratification and cleavage, are only encountered in rocks of 
sedimentary origin ; the second, joints, although common to all 
rocks, whether sedimentary or eruptive, is, it must be remarked, 
not always, or even necessarily, a parallel structure, yet it is so 
often such, that, in order to avoid confusion, it becomes neces- 
sary to consider it also under this head ; the fourth, foliation, 
is specially characteristic of the so-called metamorphic schists, 
and likewise occurs in certain eruptive rocks ; whilst the fifth or 
last, striation, is entirely confined to such rocks as have solidified 
from a previous condition of liquidity or fusion. 
Since to attempt more would be to exceed the space at dis- 
posal in a single number of this Review, it is proposed upon 
the present occasion to treat only of the three first of the 
above-named rock structures, namely, stratification, joints, and 
cleavage. 
Stratification, or, as it is very commonly called, bedding, is 
the term employed by geologists to denote a parallel structure 
in rocks caused by the successive subaqueous deposition of 
layers more or less thick of mineral matter previously held in 
solution or suspension in water, and it might even be said in 
air, for, upon a smaller scale, and in more exceptional instances, 
stratification may be of subaerial origin, as for example, in the 
cases of beds frequent in volcanic countries, consisting of alter- 
nating layers or beds of volcanic ashes, sand, and still coarser 
scoriae, which, after having been thrown up from the crater high 
up into the air, are spread out by the action of the winds over 
the neighbouring country, often to a considerable distance ; 
occasionally also, the action of the winds upon loose surface 
sands, like those in the desert, produces an arrangement of the 
particles analogous to subaqueous stratification. 
Although the planes of stratification are usually spoken of as 
parallel, this is not strictly true, for in reality they always have 
a gentle slope upwards from the sea towards that part of the 
land from which, by the action mainly of river currents, they 
have derived the rock debris, out of which the strata them- 
selves are formed. Sedimentary beds are very frequently found 
to die out or thin out like wedges, which sometimes is owing to 
a deficient supply of like material, but probably more often to 
the disturbing effects of currents, &c. 
Regarded on the large scale, however, stratification possesses 
all the general features of parallelism. 
It not un frequently happens, particularly in the case of sand- 
stones, that, independent of the usual parallelism of the beds of 
deposition to one another, another series of more or less parallel 
lines may be seen developed within the particular beds them- 
selves, and covering* them diagonally at a greater or less incli- 
