HANDBOOK OF THE METEORITE COLLECTIONS. 17 



STRUCTURE. 



As noted under the head of Classification, meteorites fall into three 

 general groups: (1) Metallic, in which the structure is due to the 

 varying crystallization of metallic alloys; (2) the stony-irons or 

 siderolites, which consist of a more or less connected mesh or sponge 

 of metal inclosing silicates, the structure of the metallic portion 

 being essentially the same as those which are all metallic; and (3) the 

 stony forms, which vary from holocrystalline or basaltic types to 

 those which are fragmental and tufaceous. It is in this last group, 

 and with particular reference to their included chondrules, that 

 meteorites depart most widely from Iniown structures in terrestrial 

 rocks. 



The crystalline structure of the purely metallic forms has been 

 sufficiently dwelt upon under the head of the Metallio constituents 

 of meteorites, and may be best comprehended by referring to Plates 

 12, 14, and 16. That of the stony-irons is shown in Plates 9 and 28. 

 It is to be noted that there are two widely distinct types of the latter, 

 one in which the included silicates have apparently undergone quiet 

 crystallization even to the extent of development of recognizable crys- 

 talline facets, and the other in which the silicates, after crystallization, 

 have become shattered and in which the metal serves as a cement 

 or binding constituent to the angular particles. This type of 

 structure or brecciation is well shown in the Admire pallasite 

 (pi. 9, fig. 2), in which the dark portions are olivine and the light 

 metal. This figure is about natural size. An enlarged portion of a 

 metalliferous area is shown in figure 1 of the same plate. In this 

 the dark outer portion is again olivine and the light (1) metal. The 

 dark interior area (3) is a spongy aggregate of iron with inclosures 

 of lawrencite and troilite. The acicular forms (4) extending into 

 this sponge are of nickel iron. Between the nickel iron (1) and the 

 spongy portion is commonly a thin plate of schreibersite (2) which 

 can not be differentiated in the illustration. 



The microscopic structure of stony meteorites of the holocrystal- 

 line type most nearly resembling terrestrial rocks of the basalt, pyrox- 

 enite, or peridotite group is shown in Plates 2 and 3. In the eukrite 

 of Juvinas (pi. 2, fig. 1) will be noted the elongated or plagioclase 

 feldspars in a crystalline granular ground of olivines and pyroxenes, 

 as in the gabbros, with metal in the interstices. In figure 2, from the 

 stone of Shergotty, the structure is more nearly that of a diabase or 

 basalt, consisting of large plates of pyroxene in a light ground, which, 

 in this case, is isotropic, the so-called maskelynite, supposed to be a 

 fused feldspar. In figure 1 of Plate 3 is shown the structure of 

 the recently fallen stone of El Nakhla. This consists of a crystalline 

 aggregate of green pyroxene and in small quantities a reddish-brown 

 5692°— Bull. 94—16 2 



