Usually lightning strikes the ground with a vertical stroke, but at times obliquely, when it traces long, horizontal lines. Often the ground may be seen turned up at the foot of trees which have been struck, the sod is torn, and stones thrown to a great distance. Sometimes, too, an excavation may be seen in the ground near the object struck, of varying breadth and height. This opening may be like a funnel or hemispherical. In a case observed on June 6, 1883, at Côte (Haute-Saône), a circular hole, having a depth of 1·20 metres, has been seen in a dyke on the declivity of the road, below a coach which was not struck. Occasionally the hole is but the beginning of a canal, hollowed rather deeply and perpendicularly in the ground, the sides of which serve as a sheath to the fulgurite. But before treating of fulgurite tubes, which constitute the most curious phenomena in the world connected with lightning, we shall discuss certain remarkable effects observed on the surface of the ground. Falling on solid rocks, the electric spark can break them, cut them, or pierce them in one or more places. Often instead of spoiling or cutting off pieces of the stone, it covers the surface instantaneously with a vitreous coat, having blisters of various colours. This vitrification is often to be seen on mountains. De Saussure found rocks of schistous amphiboles covered with vitreous bubbles, like those seen on tiles where struck by lightning. Humboldt made similar observations on porphyritic rocks at Névada de Toluca, in Mexico, and Ramond, at the Sanadoire rock in Puy-de-Dôme. In these cases, the spark, on reaching the surface, melts it more or less completely over a varying extent, and this fusion, worked upon by an extraordinary heat, produces a coat having a peculiar appearance, but in which microscopical analysis finds the elements of the body it covers. Thus the vitreous layer deposited over chalk is of chalky origin; that covering granite is of the nature of granite, etc. This does not apply to certain deposits found on rocks, and even on trees, which have been struck by lightning, and which are of very different origin. Whilst the former is only the stone in a fused or vitrified condition, the latter is caused by the presence of foreign bodies, some fragments of which have been detached by the ray and travel with it. This transport of solid substances by lightning has often been observed. Here are two examples of this strange phenomenon:--