GNEISS AND LIMESTONE CONTACT PHENOMENA 291 



On the island of Gursko, on the west coast of Norway/ the 

 writer studied a large mass of crystalline limestone surrounded by 

 an intrusive gneiss. At the boundary of the limestone there occurs 

 a zone in the gneiss consisting of a banded clinopyroxene-oligoclase 

 rock. This rock undoubtedly has obtained some of its lime by 

 assimilation of the limestone, and its characters are, in so far as 

 regards the structure and probable mode of intrusion and consolida- 

 tion during vehement movements, closely similar to those of the 

 western Massachusetts rocks. 



In all cases mentioned, acid magmas have been enriched in 

 lime derived from limestones and have crystallized as such without 

 any considerable change of composition through differentiation. 

 We shall not discuss here at all those processes by which alkaline 

 rocks are supposed to form from igneous magmas which have as- 

 similated carbonates.^ 



Diopside pegmatites in limestone regions. — While rocks, in 

 occurrence and origin similar to the diopside gneisses described, 

 are uncommon, there are in most limestone areas cut by granites, 

 very numerous dikes of pegmatite containing diopside, titanite, 

 and other lime-bearing minerals. Such pegmatites occur so com- 

 monly in the western Massachusetts area^ and elsewhere, that we 

 do not need to give any further examples. The large quantity of 

 titanite present in many of these dikes is remarkable. 



The writer's field evidence from southern Finland goes to prove 

 that the assimilation of lime has taken place within the pegmatite 

 fissures themselves and not in the granitic parent magmas of the 

 pegmatite. This is well illustrated by the frequently observed 

 feature that a pegmatite, cutting through limestone and other 

 rocks as well, has developed much diopside and titanite only while 

 intersecting the limestone, but is an ordinary mica pegmatite 

 outside of the limestone."* 



^ P. Eskola, "On the Eclogites of Norway," Vid. selsk. Skri. Mat.-naturv. Kl.I, 

 No. 8 (1922), p. 24. 



^ As the writer has suggested, the Sviatoy Noss rocks show an incipient stage of 

 development toward "alkalinity." 



3 B. K. Emerson, U.S. Geol. Surv., Bull. 597, p. 19. 



'' Cf. A. Laitakari, op. cit., p. 7. 



