Page:Popular Science Monthly Volume 37.djvu/386

372 fragment of rock on top of the snow, which is supposed to be part of the meteorite.

All meteorites appear to be fragments broken off from larger bodies. Sometimes numerous fragments reach the earth, and at other times only single masses. Thus, in the fall at L'Aigle, already mentioned, about three thousand pieces were picked up, scattered over an ellipse more than six miles long. An equally large number fell at Knyahinya, June 9, 1866. Still more at Pultusk in 1868. Several thousand were also picked up after a fall at Estherville, Emmett County, Iowa, May 10, 1879. In such a rain of meteorites the fragments vary greatly in size, some weighing less than a grain, while the largest may weigh a hundred pounds or more. In most cases the peculiar characters or composition of the various specimens make it easy to recognize them as fragments of the same mass. In the case of the Estherville meteorite most of the pieces were coated with a fused crust, owing to the explosion having taken place before they had lost their great velocity.

In the case of a stone which fell at Butsura in 1861, fragments found three or four miles apart could be fitted together, and some of the pieces, though fitting perfectly, had been coated on the faces of juncture with a thin crust, showing that they had been blown apart when the meteorite was still very high in the air.

Meteorites, when not seen to fall, are easily recognized, not only by the characteristic fused crust and pittings already referred to, but by certain very marked peculiarities of structure. There are three large groups: those consisting of metallic iron; those consisting of earthy minerals containing only grains of metallic iron; and those like the Pallas, made up of a continuous network of iron inclosing stony matter. The stony meteorites are usually made up of little rounded grains imbedded in a ground-mass of fragments of the same material, a type of structure called chondritic, which in its details is so characteristic that pieces of the same mass can usually be easily identified, even though found at places or times remote from each other. The iron meteorites are still more easily recognized, although only about nine at most have been seen to fall; for, since iron has not been found in masses of any size in terrestrial rocks, unless in Greenland, these large meteoric fragments are at once noticed wherever found. Stony ones, on the contrary, are not only apt to be overlooked, but the falls of past ages must have been altered and broken up by weathering. Meteoric iron can be easily identified, because it is usually extremely malleable, but at the same time very tough, owing to its being made up of a network of crystalline plates, the plates consisting of pure iron, bounded by layers of an iron-nickel alloy and other impurities, which have separated