COMPARISON WITH OTHER REGIONS 415 



tioned. Similarly, among the more basic rocks we find many which 

 correspond both chemically and mineralogically to those of Magnet 

 Cove. There are also. analogies with the borolanite of Teall* and the 

 malignites of Lawson,f though in both of these K 2 is relatively much 

 higher. 



DIFFERENTIA TION ALONG TWO LINES 



A study of the rocks of all these regions indicates that magmas rich 

 in soda tend to differentiate toward the basic end along two lines. 

 Along one there is an accumulation of Na 2 0, giving rise to such peculiar 

 types as the urtite of Kola with 17 Na 2 0, the sodalite-syenite of Kang- 

 erdluarsuk with Na 2 up to 14.7, and the sussexite of Beemerville with 

 Na 2 11. Along the other line iron oxides and lime % tend to accumu- 

 late, giving rise at the basic end to the basic segregations of Serra de 

 Monchique, the jacupirangites of Brazil, and similar rocks of Alno. At 

 Magnet Cove the first branch seems to be absent, since nothing analo- 

 gous to the urtites, etcetera, is found there; but, on the other hand, the 

 iromlime branch is represented by the ijolites and jacupirangites. 



Summary 



The structure of the complex is briefly described, and from the evi- 

 dence of form of area, relations to surrounding shales, the presence of 

 an overlying zone of metamorphosed rocks, the arrangement and serial 

 petrographical and chemical characters of the main types, together with 

 other minor points, it is shown that the igneous complex is probably a 

 laccolith, and certainly a unit or integral mass of intruded magma. The 

 component abyssal types are not due to successive injections, as was 

 suggested by J. F. Williams, but are the products of a differentiation 

 in situ of the originally homogeneous mass of intruded magma (" lacco- 

 lithic differentiation ,; of Brogger). 



The main rock types are briefly described, some new analyses being 

 given, and they are shown to form a regularly graded series, ranging 

 from foyaite, through leucite-porphyry, shonkinitic syenite, normal 

 ijolite, and biotite-ijolite, to jacupirangite. This serial and common 

 genetic character is shown both mineralogically and chemically. It is 

 probable that the dikes of tinguaite and nepheline-porphyry are aschistic, 

 while those of the monchiquitic rocks are diaschistic. 



*Horne and Teal! : Trans. Roy. Soc. Edinb., vol. xxxviii, 1893, p. 163. 

 | Lawson : Bull. Dept. Geol. Univ. Cal., vol. i, p. 337. 



% MgO seems to be associated with K 2 when it is abundant together with either of the two 

 alkalies. 



