24 BULLETIN 149, UNITED STATES NATIONAL MUSEUM 



of the original structure, whether crystaUine, glassy, or fragmental. 

 This feature has caused a great diversion of opinion among students. 

 As will be observed, the present writer bases his conclusions as to 

 the original clastic (fragmental) nature of the chondritic varieties not 

 on structure alone, but on the presence in close association of one 

 and the same mineral under varietal forms of crystal development 

 such as are seemingly impossible products of direct cooling from a 

 molten magma. 



Few meteoric stones, probably not over a score of those now known, 

 show the crystalline structure characteristic of terrestrial igneous 

 rocks, either basalts or peridotites. In the prevailing system of 

 classification they are divided into two general groups. I. Calcium- 

 aluminum-rich stones nearl}'^ free of nickel iron and without chon- 

 drules, and II, magnesium-rich stones likewise nearly free of nickel 

 iron and nearly or completely free of chondrules. Group I is again 

 subdivided into (1) the angrites — of which but a single representative 

 is known, which consists mainly of a dark brownish augite and a 

 little olivine iron sulphide and with a crystalline granular structure; 

 (2) the euJcrites, which consist essentially of augite and anorthite 

 with iron sulphide and also a crystalline granular structure like many 

 dolerites; (3) the sJiergottites of which there is also but a single 

 representative known, which consists of augite and the isotropic 

 feldspar maskelynite and a little magnetite, with likewise a crystal- 

 line granular structure; and (4) the howardites, consisting of augite, 

 anorthite, bronzite, and olivine in a tufFaceous ground with some- 

 times eukritic segregations. 



Group II is likewise subdivided on mineralogical and structural 

 grounds into (1) the hustites, of which but a single representative is 

 known, which consists essentially of diopside and bronzite with 

 smaller quantities of oldhamite, plagioclase, nickel iron, and osbornite, 

 with a nearly crystalline structure; (2) the chassignites consisting of 

 an iron-rich olivine and small quantities of chromite and with a 

 crystalline granular structure, of which type but a single stone is 

 known; (3) the cJiladnites, consisting of a crystalline granular aggre- 

 gate of a rhombic pyroxene; and (4) the ampJioterites consisting of 

 olivine and bronzite with smaller quantities of sulphide and nickel 

 iron. The structure is sometimes granular and sometimes chondritic. 



The microstructure of the eukrites and howardites has been 

 described in detail by various writers, including Tschermak, Wahl, 

 Berwerth, and others, all of which have been the subject of review 

 by Lacroix ^^ to whose work the reader is referred for details. The 

 Bereba eukrite is described as a breccia of doleritic fragments cemented 

 by recrystallized finely pulverulent material of the same mineral 

 nature. The essential constituents are pyroxene and anorthite with 



" Arch, du Museum D'Historie Naturelle, vol. 1, ser. 6, 1926. 



