But it is not only in abandoned mines that falls of roof take place, nor yet alone at the unpropped face of breast or gangway. They are liable to occur at almost any point in any mine. Sometimes only a small piece of slate, not larger perhaps than a shingle, will come down; again the roof of an entire chamber will fall. It is possible that two or more chambers will be involved in the disturbance, and instances occasionally occur in a working mine where a fall covers an area many acres in extent. The falls that are limited in extent, that are confined to a single chamber or the face of a chamber, do not interfere with the pillars and can be readily cleared away. They are due to lack of support for the roof, to insufficient propping and injudicious blasting, and may, to a great extent, be guarded against successfully by care and watchfulness. But to foresee or prevent the more extended falls is often impossible. They are due to the general pressure of overlying strata over a considerable area, and both props and pillars give way under so great a strain. Sometimes they come without a moment’s warning; usually, however, their approach is indicated by unmistakable signs days or even weeks in advance of the actual fall. There will be cracks in the roof, small pieces of slate will drop to the floor, the distance between floor and roof will grow perceptibly less, pillars will bulge in the middle and little fragments of coal not larger than peas will break from them with a crackling sound and fall to the floor, until a deposit of fine coal is thus formed at the base of each pillar in the infected district. This crackling and falling is known as “working,” and this general condition is called a “crush” or a “squeeze.” If one stands quite still in a section of a mine where there is a squeeze, he will hear all about him, coming from the “working” pillars, these faint crackling noises, like the snapping of dry twigs under the feet. Sometimes the floor of underclay or the roof of shale is so soft that the pillar, instead of bulging or breaking, enters the strata above or below as the roof settles. When this occurs it is called “creeping.” In the steep-pitching veins the tendency of the pillars on the approach of a squeeze is to “slip,” that is, to move perceptibly down the incline. When these indications occur the workmen are withdrawn from the portion of the mine which is “working,” and vigorous measures are taken to counteract the pressure, by props and other supports placed under the roof. Sometimes this work is effectual, sometimes it is of no avail whatever. Often the fall comes before the first prop can be set; and when it comes the crash is terrible, the destruction is great. However, not many feet in thickness of the roof strata can come down; the slate and rock which first fall are broken and heaped in such irregular masses on the floor that they soon extend up to the roof and afford it new and effectual support.