14 
Fig. 4 represents strata of rocks tilted up into an inclined position. The angle formed by the 
inclined surface a b and the horizon is called the dip of the stratum; and the course of the edge of 
the stratum, de, is the strike. The dip and strike are therefore at right angles with each other; 
that is, if the dip be towards the Kast, the strike will be North or South. The angle 6 a ce, which 
is about 43°, represents the dip of the surface a 6, and we say the rock dips 43° West. 'Trifling as 
this fact may seem, practically it is of the utmost value. When barely the edge of a bed of ore 
can be seen on the surface, it is obviously of great importance to be able to determine in which 
direction it dips, and even the amount of that dip. Otherwise, in sinking our shaft for exploration, 
we may locate it in an unfortunate position, or miss the bed altogether. 
In York District I saw a bed of iron ore which had once been worked. A new shaft became 
necessary: it was sunk, but no ore found. It was therefore concluded that the bed was exhausted: 
further search was accordingly given up, and the ore remained to be re-discovered some years 
after. A knowledge of the simple facts just explained, would have prevented the vexation and loss 
incurred here. ‘The new shaft, in consequence of a change in level of the surface, was sunk on 
the wrong side of the strike. 
In tracing the edge of a stratum or bed over a considerable space, the undulations of the surface 
of a country must be taken into account; otherwise we may be led into great errors. The line 
towards which strata lean, or that along which the force acted, which tilted them up, is called the 
anticlinal axis. In Fig. 4, de vepresents the direction of this axis—which 1s the line towards 
which the strata a 6 and f g are inclined. The term synclinal is applied to the line from which 
contiguous strata incline: it is the line joining the bases of the strata fg and g h. 
Out crop is another term frequently used in speaking of rocks. It is applied to the edges of 
rocks, as they rise up or are exposed at the surface, when they are said to crop oud. 
When superincumbent beds rest without any conformity to the stratification of the lower beds, 
they are said to rest upon them unconformably. The beds a bc, Fig. 5, represent unconformable 
beds resting on the upturned edges of the strata 1 23 45. 
