GEOGNOSY AND GEOLOGY. 12] 
7. Stratification. 
Stratification, which always extends over greater distances than true 
cleavage, is peculiar to rocks deposited from water. Plutonic rocks are 
sometimes subdivided in a manner bearing a great resemblance to stratifi- 
cation ; the affinity in structure is, however, only apparent, not real, the 
subdivisions being merely a tabular cleavage of columns or parallelopipedons. 
Granite often presents this appearance, as shown in pl. 43, fig. 12, where 
the corners and edges of the separated portions have been rounded off and 
weathered away by atmospheric agencies. Stratification and cleavage 
may occur together, as is often seen in slate rocks, whose cleavage planes, 
parallel constantly to each other, intersect the planes of stratification at all 
angles. 
The portion of rock included between two planes of stratified separation 
is called a stratum. The thickness of strata is exceedingly variable, and 
not unfrequently immense beds are found to alternate with quite thin 
layers. | . 
The planes of stratification are generally straight, although not always of 
great extent. At times their general direction is straight, with occasional 
undulations, curvings, and contortions (pl. 43, fig. 8). These bendings 
and foldings, which sometimes give rise to the formation of caves (as in the 
grotto of Jupiter on the Island of Naxos, pl. 51, fig. 8), often run into the 
finest crumpling, as may frequently be observed in clay siate, this rock 
being, for other reasons, especially adapted to the study of stratification, 
Silicious slate is not unimportant in this respect, a complicated stratification 
being peculiar to it. The strata are often entirely curved (pl. 58, fig. 8, 
representing clay slate strata on the coast of Scotland, and pi. 43, fig. 9, 
strata on the coast near Wapness, not far from Guns-Green), this condition 
being more interesting than a partial flexure. There are two principal 
differences in this respect, according as the opening of the bend is above 
(pl. 43, fig. 5) or below (fig. 7); as also, whether the bend be arched or 
angular like the roof of a house. When the opening is turned up we have 
a trough: when below, a saddle. Troughs and saddles generally succeed 
each other, as seen in great perfection in a section of Brittany, between 
Rennes and Nantes (pl. 46, fig. 3). 
Strata frequently exhibit a change in their position, so that one part of 
the same layer stands at a higher or lower level than another. When the 
variation is inconsiderable it is called dislocation; where of greater 
amount, displacement. These frequently stand in such connexion with 
veins and fissures as to render it not unreasonable to ascribe all to the 
same system of forms. 
8. Arrangement of Strata. 
The position of planes of stratification is either horizontal, vertical, or 
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