PETROLOGICAL ABSTRACTS AND REVIEWS 409 



regard for the injection-metamorphism that borders the opdaUte-trondhjemite 

 type of rocks. The clayey sediments have undergone regional metamorphism 

 ranging in its products from the mere development of good cleavage, through 

 the development of garnet-mica-schists, and finally the growth in these of basic 

 plagioclase and of pyroxenes. 



Most distantly from the borders of the acid masses the phyllite becomes 

 altered to garnet; farther in toward the intrusive this is replaced by biotite, 

 thus presenting an order that is the reverse of that more commonly observed. 

 There is also a progressive increase in sodium content as the intrusions are 

 approached. Closer yet microperthite makes its appearance. These rocks 

 are in turn altered to injected mica-schists and gneissic mica-schists; the 

 "injected" schists are not so in the sense that a liquid magma has been forced 

 inward along the lines of schistosity; injection, as here used, implies rather a 

 metasomatism, with a relatively small addition of extraneous material. 



The upper Silurian green slates, too, form significant "injection schists." 

 Even the basic differentiates in the opdalite-trondhjemite series have under- 

 gone some contact alterations, because affected by the later acidic intrusions. 



Various types of metamorphic rock are described in detail; these include 

 quartz-muscovite-chlorite-phyllite, quartz-muscovite-chlorite-garnet-phyllite, 

 quartz-muscovite-biotite-garnet-phyllite, chloritoid-slates, and albite-porphyro- 

 blastic schists. Transitions between these and "augengneisses" are also 

 known. When acid material is added by injection, the albite-porphyroblastic 

 rocks are altered to injection- (injected) gneisses. These are partly bedding- 

 plane injection types, with marked parallelism in the texture of the crystalline 

 injected rock, partly vein-injected gneisses, and augen-gneisses (which are 

 attributed also to injection of clay slates by the more acid rock). 



Much of the alteration is due especially to the addition of quartz, water, 

 and soda. These may have been brought in as soluble sodium silicate. 



The metamorphism here described is compared with that elsewhere in 

 Europe and also with that in the Christiania District, which has been studied 

 in detail. In the latter instance it is interesting to note the far greater regional 

 distribution of the contact phases, attributed by Goldschmidt to the greater 

 water content of the effective magmas. In the Stavanger District probably 

 the most conspicuous thing is the extensive contact metamorphism of siliceous 

 sediments. Into the same class of metamorphism, due essentially to chemical 

 differences between the country rock and the "magma milk," falls that de- 

 scribed by Brauns under the name of "pyrometamorphosis." 



Gordon, Samuel W. "Ordovician Basalts and Quartz Diorites 

 in Lebanon County, Pennsylvania," Proc. Acad. Nat. Sci., 

 Philadelphia, Nov., 1920. Pp. 354-57, figs. 5. 

 Includes the first report of Paleozoic volcanic rocks in the form of a basalt 



flow, from Lebanon County, Pennsylvania. Intrusive into the Martinsburg 



