PETROGRAPHICAL ABSTRACTS AND REVIEWS 187 



Nordenskjold, Ivar. "Der Pegmatit von Ytterby," Bull. 

 Geol. Inst. Univ. Upsala, IX (1910), 183-228. 

 Numerous lenses of pegmatite occur at Ytterby on Resaro Island, 

 about 20 km. E.N.E. of Stockholm. Some of the pegmatites are found 

 between diorite and gneiss, and others occur in hornblende gneiss. A 

 zonal structure is noticeable, the pegmatites being finest grained near 

 the contact. Large masses of pure red potash feldspar (microcline 

 perthite), white plagioclase (oligoclase), and massive quartz are found 

 in the center of the lenses. The potash feldspar is especially valuable 

 and the minerals are mined and used in the manufacture of porcelain. 

 Graphic granite is also abundant. Of the micas, a dark biotite is more 

 common than muscovite. It is often chloritized and it is with this 

 altered mica that the rare minerals fergusonite, gadolinite, etc., are found. 

 The descriptions of the rare earth minerals, largely historical, include 

 also yttrotantalite, allanite, xenotime, and altered zircon. 



W. T. Schaller 



Rastall, R. H. "The Skiddaw Granite and Its Metamorphism," 

 Quart. Jour. Geol. Soc. (London), LXVI (1910), 1 16-41. Map. 

 A study of the alteration produced in the sedimentary rocks of 

 the Skiddavian Series by the intrusion of an alkali granite commonly 

 known as the Skiddaw granite. The metamorphism extends over a 

 considerable area, although the outcrops of granite are limited to three 

 of rather small extent which the author supposes to be part of a large 

 mass continuous beneath the surface. From the repetition of the same 

 sequence of rock-types in reverse order, it appears that the structure of 

 the region is that of a complicated anticline or syncline, the former 

 being most probable. The position of the granite mass suggests that 

 it was intruded along the main axis of this anticlinorium, and the author 

 believes its injection closely followed or even accompanied the folding. 

 If this is true, here is an example of a direct relation between intrusion 

 and folding. The chief minerals produced by the metamorphism were 

 cordierite, andalusite, biotite, and muscovite, with garnet and staurolite 

 near the granite contact. The absence of cyanite and sillimanite indi- 

 cates that the rocks were never subjected to a very high temperature, and 

 all the evidence points to the maintenance of a moderate temperature 

 for a long period of time, such as would result from the intrusion, under 

 a thick cover, of an igneous mass not very highly heated. 



Albert Johannsen 



