chain?"

"Yes, there! The coal seam jumps upwards fifty feet. That's why the cars, after rolling down nearly a quarter of a mile, by gravity, have to be pulled up fifty feet by an endless chain, to rejoin the same seam and then to go rolling on down by themselves."

"Just what are faults?"

"H'm, that's a bit hard to explain to you, Anton, because you don't know anything about geology, but maybe I can get you to see. Faults are breaks in the layers of rock, or in the stratification, as it is called. All coal seams and the rocks above and below them have been laid down by water. Since water levels everything, these layers of rock were level, once.

"In ages past, however, the crust of the earth changed a good deal. As the crust cooled, it contracted, crumpling up these different layers into all sorts of shapes. Sometimes it bulged them up, sometimes it hollowed them down so that the edges rose. Quite often a layer of rock would be cracked right across, and one half would stay level while the other shot up almost a right angle. A good many mountains show the result of this, and if you look at such rocks as are sticking up out of the ground you will see some of them standing right on edge. Once in a while, part of the broken crust slid over the other part. Then, too, though the surface may not always show it, there have been breaks in the strata below, and at the break, the layer has sunk or risen quite a distance from its former level.

"If that happens to a coal seam, you can see that where the seam breaks, suddenly, the rest of it will continue on another level, perhaps only a few feet higher or lower, perhaps a good deal more. It's up to the mine geologist to find where the coal has gone to, and it's the business of the mine engineer to remodel the entire system of working the mine in order to get at that seam."

"And are all coal mines mixed up in that funny way?" Anton queried.

"Most of them. Oh, there's no end to the tricks a coal seam can play. A deep coal seam may split into two narrow ones, too thin to work. The whole seam may quickly dwindle away to nothing, showing that, in ages past, a river came rolling over it and washed away all the forest bog. Sometimes, especially with the lowermost seams, the forest grew on rolling land, so that the bottom of the coal seam is irregular, causing all sorts of trouble in laying rails for the cars to roll on. Sometimes the layer of rock under a coal seam is so soft that when you start to timber it, the timbers sink into the floor and the roof comes toppling down.

"Among the queerest of all the things a mine geologist strikes are what are called dykes. These are great shafts of igneous rock, which were thrust up from the interior of the earth in a white-hot state and which burned away the coal as they rose. They put a dead stop to a working. I could tell you a dozen more freak things that a coal seam can do. A mine geologist has not only a new problem to tackle with every mine, but, often, with every mine gallery."

"Is that what you're studying to be, Clem?"

"No, indeed!" The young fellow's answer was emphatic. "That's 'way out of my reach. It takes a college man, with special technical training and a big experience, to be anything of a mine geologist. All I'm trying to do is to learn enough about it so that when I get to be a mine boss—if I ever do—I'll know what my chiefs are trying to do and I'll be able to help them.

"Take Otto, for example. There isn't a better worker in the mine. He gets out more coal and less broken stuff than any other man below ground. But he'll never be anything but a hewer, because he doesn't want to learn. Why, just the other day, he was growling because the mine was shut down to repair one of the shafts, though the other shaft was working all right."

"So were a lot of the men," Anton put in. "Why couldn't they go on working, with one shaft?"

"Against the law," was the crisp answer. "That's the A B C of mining. And I'll show you why! All mines are required to have two shafts, in case of accident. That law was passed because of a famous disaster that happened in England nearly a hundred years ago.

"In those days, colliers had only one shaft. One day, the beam of an engine which was directly over a shaft snapped, and a huge piece of machinery, weighing several tons, tumbled into the shaft and stuck, not far from the bottom. As it fell, it ripped away the planking which lined the shaft and a whole lot of loose rock and earth fell on top of the piece of machinery, blocking up the shaft entirely and stopping any air from passing. There were over two hundred men and boys at work below ground.