AGENCIES OF METAMORPHISM 407 



That a rind, or border, indicative of a second rise in temperature is 

 to be found about some of the chondrules of the Grosnaja stone was 

 noted by Tschermak. This stone is, however, a black chondrite, which 

 feature in itself is considered as due to heat. More enigmatical are such 

 forms as are shown in figure 5, plate 4, from the Beaver Creek chondrite, 

 where there is an irregular outer border strongly suggestive of a secondary 

 groM^th. If such is the case, however, it is questionable if the same must 

 not have taken place at some prior, earlier, stage of meteorite history, 

 since the stones are both almost wholly unaltered spherulitic ("Kugeln") 

 chondrites.^** 



Tschermak, in his description of the Alfianello and Mocs stones,^** the 

 first an intermediate and the second a white chondrite, states that in the 

 Alfianello he finds both isotropic and doubly refracting granules, and 

 some that appear but partially isotropic, transition phases, as it were 

 (figure 1, plate 5) : 



"Dies fiirht dazu, die isotropen Korner als umgeschmolzenen, also durch 

 Ertiitzung isotrop gewordenen Plagioklas aiizusehen." 



In Mocs, plagioclase occurs in the body of the stone, but no mas- 

 kelynite. In the fused crust, maskelynite alone is found. 



H. Michel,^^ whose excellent work on the feldspars of meteorites is 

 most helpful, notes the indications of thermo-metamorphism in the 

 Juvinas stone, a eukrite, and that the pyroxenes have become more or 

 less granulated and filled with glass inclusions. The clear border or 

 zones about the plagioclase, regarded as due to this cause, are free from 

 inclusions, less strongly doubly refracting than elsewhere, and have 

 variable angles of extinction. He thinks that this alteration was brought 

 about at a temperature lower than that of the fusion points of the 

 pyroxenes (1265°), resulting only in a reduction of the feldspars to a 

 condition of viscosity. He notes further that the white chondrites are 

 rich in plagioclases which are lacking in the black, and that the inter- 

 mediate and gray chondrites occupy a position in this respect interme- 

 diate of the two. This indirectly confirms Meunier's opinion as to the 

 origin of these black forms. 



Accepting the fusion points as given by Doelter^- (bronzite, 1310°- 

 1400°, and anorthite, 1250°), it is not difficult to conceive of metamor- 

 phism due wholly to heat, as advocated by Tschermak and as is appar- 



^ I do not agree with Brezina (Die Meteorltensammlung, 1895) in classing this Beaver 

 Creek stone as a crystalline spherulitic chondrite (Cck). 

 3" Die Meteoriten, etc. 



31 Tschermak's Min. und Pet. Mitteil.. vol. 31, 1912, pp. 650 et al. 

 *= Handbuch der Minerchemie, vol. 1, pp. 658-659. 



XXIX— Bull. Geol. Soc. Am., Vol. 32, 1920 



