Page:Popular Science Monthly Volume 30.djvu/558

538 Dr. Gilbert describes the fulgurites found by Humboldt upon the high peaks of the Nevada de Toluca, in Mexico. The rock on which they were found is described as a trap porphyry taken from the summit of the Pic del Fraile at a height of 2,364 "toisen" (about 4,621 metres). Masses some two feet square are said to occur covered with a thin layer (about one half of a millimetre) of a pistacio-green glass. In one instance, where a feldspar crystal intervened, the glass was white. The peculiar shimmer of this glass in the sun led Humboldt to ascend this precipitous peak even at the risk of his life. In many places the rock was completely pierced by small cylindrical tubes, which were lined with the same greenish glass, and this was found to resemble closely the glass from the inner walls of the tubes found at the Senner Heide.

The most interesting and, so far as I am aware, the only investigation of this kind undertaken in America is that of Mr. J. S. Diller on fulgurite from Mount Thielson, in Oregon. The summit of this mountain is described as being composed of a hypersthene basalt, very precipitous and difficult of ascent. The fulgurite occurs both as a superficial coating on the rock, and in the form of tubes. "Although spread over a considerable surface, it is not evenly distributed, but is arranged in patches of drops and bubbles of glass in very much the same way as paint which has been put upon a greasy surface." The glass is described as translucent and of a greenish color. In places the tube penetrated the rock to a depth of a few inches, having a diameter of from 10,5 to 20 millimetres. These have a glassy lining of some two millimetres in thickness. The tubes are not regarded as having been produced by the lightning itself, but to be pre-existing tubes and cavities (the rock is naturally spongy), into which the fluid passed and fused the walls. The chemical composition of this glass was found to be silica, 55,04 per cent; alumina and iron, 28,99 per cent; lime, 7,86 per cent; magnesia, 5,85 per cent; potash and soda not determined; loss by ignition, 1,11 per cent; total, 98,85 per cent. This was also found to be practically the composition of the groundmass, or non-crystalline portion of the rock.

The fulgurite found on the summit of Dom du Gonte, one of the peaks forming a part of the chain of Mont Blanc, has been recently described by Rutley. The rock is represented as a hornblendic gneiss. The fused material formed merely a superficial coating in the form of attached globules or irregularly fused pellets and blotches of brownish, black, or white glass. The white glass resulting from the fusion of the feldspar, while the dark in like manner was of hornblendic origin. This occurrence of the two glasses, unmixed even