514 Alexander Scott — The Crawfordjohn Essexite. 



Analyses 2 and 3 represent two basic monchiquites, and the fact that 

 the Craighead rock is intermediate in composition between these two, 

 shows a close relationship to this group. A recalculation of 

 analysis 2, after deduction of the amount of C0 2 , would bring it 

 fairly close to analysis 1. A comparison of columns 1 and 4 indicates 

 a high degree of affinity with the nephelite basalts. Several 

 occurrences of the latter have been noted among the Carboniferous 

 lavas and intrusives of the Midland Valley, 1 where they exhibit some 



TABLE III. 



1. Monchiquite, Craighead, analyst A. Scott. 



2. Monchiquite, Mile End, Montreal (includes -47 Fe S 2 ), analyst M.F.Connor. 2 



3. Monchiquite, Pulaski Co., Arkansas, analysts Noyes & Bracket. 3 



4. Average of ten analyses of nephelite-basalt. 4 



5. Camptonite, Maena, Norway, analyst L. Schmelk. 5 



6. Ijolite, Ambaliha, Madagascar, analyst Pisani. 6 



similarity with the Hillhouse type, 7 merely differing from it in the 

 presence of nephelite and the smaller amount of olivine. Since the 

 Hillhouse basalts maybe regarded as the microporphyritic equivalents 

 of the Craiglockhart type, it is significant that, while the Craighead 

 aphanites are related to the former, the essexites are not far removed 

 from the latter. 8 Analyses 1 and 6 resemble each other so closely 

 that the Madagascar ijolite may be regarded as a plutonic equivalent 

 of the rock under consideration. 



The microscopical and chemical examinations of the Craighead 

 rocks give some clue as to the nature of the differentiation. The 

 augite of the marginal rocks is zoned to a much greater extent than 



1 E. B. Bailey in Geology of East Lothian (Mem. Geol. Surv.), 1910, p. 99, 

 pp. 105-13. 



2 Geological Congress, Canada, 1913, Guide-book No. 3, p. 46. 



3 Quoted in Iddings, Igneous Rocks, vol. ii, p. 413, 1913. 



4 Compiled from H. Kosenbusch, Elemente der Gesteinslehre, 3rd ed., 1908, 

 and J. P. Iddings, Igneous Bocks, vol. ii, 1913. 



e W. C. Brogger, loc. cit., p. 26. 



6 A. Lacroix, loc. cit., p. 138. 



7 Cf. J. S. Flett, loc. cit., p. 316 ; E. B. Bailey in Geology of Glasgow 

 District (Mem. Geol. Surv.), 1911, p. 138. 



8 Cf. E. B. Bailey, loc. cit. 



