COMPOSITION AND STRUCTURE OF METEORITES 45 



assumption that the shock producing the fracture took place some- 

 where in space where the metal had become so cold as to be brittle. 



The black coating on the surface of the stony meteorites is, as 

 already noted, a more or less perfect glass, due to the fusion of the 

 various constituents from the heat generated during the passage of 

 the stone through the atmosphere. This, as shown in thin section 

 (see pi. 28, fig. 1) is never of more than a few millimeters in thickness. 

 It consists, in the case of the Allegan stone figured, of a black glass 

 interspersed with numerous residuary particles of metal and unfused 

 silicates, which passes gradually into the unaltered granular stone. 

 Sections of the thick, blebby glass from the lower surface show air 

 vesicles and numerous crystallites imperfectly secreted from the 

 glassy base, and too small to be seen in the figure, together with the 

 residuary unfused particles of the original minerals. 



The so-called "black chondrites" are considered by Meunier and 

 others as chondrites of the ordinary white, gray, or Icugeln type which 

 have been heated to a temperature considerably short of that of fusion, 

 as already noted. 



METEORITES COMPARED WITH TERRESTRIAL ROCKS 



From what has been written it must be evident that, though com- 

 posed of the same elemental matter, meteorites dift'er in some very 

 marked respects from terrestrial rocks. Nevertheless there are 

 resemblances, some of which, when one considers the problematic 

 source of these bodies, are of peculiar interest. 



All known meteorites are composed of volcanic materials, and none 

 has shown any traces of animal or vegetable life, unless the carbona- 

 ceous matter is to be so considered. This, however, is a wholly 

 unnecessary and, indeed, unwarranted assumption. It is true that 

 the German, Otto Hahn, when the possibilities of the microscopic 

 study of rocks were first becoming realized, described as organic 

 (corals and crinoids) what are now known to be but incipient crystal- 

 lizations of silicate minerals. Nothing in the nature of a terrestrial 

 sedimentary rock, a sandstone, shale, or limestone, or a metamorphic 

 like a schist or gneiss, has yet, so far as known, come to us from space, 

 nothing of a pumiceous nature, and nothing in content of silica, 

 alumina, lime, or alkalies corresponding to the granites (the tektites, 

 if meteoric, would most nearly correspond to this type of terrestrial 

 rock) and nothing of the nature of a true vein rock. Further, and 

 this seems the more singular when theories of earth history are 

 considered, nothing that can with certainty be ascribed to a meteoric 

 origin has been found in terrestrial beds of any geological horizon 



