V8 
ELEMENTS OE GEOLOGY. 
a larger space, so-as to press upon another rock, which, if flex¬ 
ible, may be squeezed into a bent and folded form. It will 
also appear, when the volcanic and granitic rocks are de¬ 
scribed, that some of them have, when melted in the interior 
of the earth’s crust, been injected forcibly into fissures, and 
after the solidification of such intruded matter, other sets of 
rents, crossing the first, have been formed and in their turn 
filled by melted rock. Such repeated injections imply a 
stretching, and often upheaval, of the whole mass. 
We also know, especially by the study of regions liable to 
earthquakes, that there are causes at work in the interior of 
the earth capable of producing a sinking in of the ground, 
sometimes very local, but often extending over a wide area. 
The continuance of such a downward movement, especially 
if partial and confined to linear areas, may produce regular 
folds in the strata. 
Creeps in Coal-mines. —The “ creeps,” as they are called in 
coal-mines, afibrd an excellent illustration of this fact.— 
First, it may be stated generally, that the excavation of coal 
at a considerable depth causes the mass of overlying strata 
to sink down bodily, even when props are left to support the 
roof of the mine. “ In Yorkshire,” says Mr. Buddie, “ three 
distinct subsidences were perceptible at the surface, after the 
clearing out of three seams of coal below, and innumerable 
vertical cracks were caused in the incumbent mass of sand¬ 
stone and shale which thus settled down.”* The exact 
amount of depression in these cases can only be accurately 
measured where water accumulates on the surface, or a rail¬ 
way traverses a coal-field. 
When a bed of coal is worked out, pillars or rectangular 
masses of coal are left at intervals as props to support the 
roof, and protect the colliers. Thus in Fig. 59, page 79, rep¬ 
resenting a section at Wallsend, Newcastle, the galleries 
which have been excavated are represented by the white 
spaces (X, 5, while the adjoining dark portions are parts of 
the original coal seam left as props, beds of sandy clay or 
shale constituting the floor of the mine. When the props 
have been reduced in size, they are pressed down by the 
weight of overlying rocks (no less than 630 feet thick) upon 
the shale below, which is thereby squeezed and forced up 
into the open spaces. 
Now it might have been expected that, instead of the floor 
rising up, the ceiling would sink down, and this efiect, called 
a “ thrust,” does, in fact, take place where the pavement is 
more solid than the roof. But it usually happens, in coal- 
* Proceedings of Geol. Soc., vol. iii., p. 148. 
