Speaking of the practical teaching of geology, in his address in the British Association, Prof. A. H. Green took np the case of places where it is hard to find within manageable distance of the school the kind of field geology which is within the grasp of a beginner. Even here the teaching need not be wholly from books. Object-lessons may be given indoors. "For instance, give a lad a lump of coarsest sandstone; let him pound it and separate by elutriation the sand-grains from the clay; boil both in acid, and dissolve off the rusty coating that colors them; ascertain by the microscope that the sand-grains are chips and not rounded pellets, and so on. All such points he will delight to worry out for himself; and, when he has done that, an explanation of the way in which the rock was formed will really come home to him. Or it is easy to rig up contrivances innumerable for illustrating the work of denudation. A heap of mixed sand and powdered clay does for the rock denuded; a watering-can supplies rain; a trough, deeper at one end than the other, stands for the basin that receives sediment. By such rough apparatus, many of the results of denudation and deposition may be closely imitated, and the process is near enough to the making of mud pies to command the admiration of every boy. . . . The great facts of physical geology, which have so important a bearing on geology and history too, often admit of experimental illustration, such, for instance, as the well-known methods of imitating the rock-folding caused by earth-movements."