\ 
\ 
F. Dixey—The Magnesiun Group of Igneous Rocks. 489 
7. Haliburton and Bancroft Areas, Canada.t 
8. The Cortlandt Area, New York.’ 
9. The Blue Ridge, Virginia.* 
10. State of Bahia, Brazil.4 
11. Southern Eyre Peninsula, Australia.® 
12. Adelie Land, Antarctica.° 
It may be mentioned at this point that the North-West Highlands 
of Scotland,’ although lacking in typical development of the rocks 
of the Magnesian Group, is nevertheless related to many of the 
above areas in several important respects, particularly in the 
prevalence of amphibolites of various kinds. 
IV. Aare or THE MAGnesIAN GROUP. 
It will at once be noticed that the areas given in the preceding 
paragraph include the more important known outcrops of pre- 
. Cambrian igneous and metamorphic rocks, and attention may here 
be drawn to the remarkable lithological similarity of the pre- 
Cambrian terranes, a similarity which has already been noticed by 
several writers. Tbus it may reasonably be concluded, in spite of 
the abundance of pre-Cambrian granitic rocks, that rocks of the 
Magnesian Group were of much greater importance in pre-Cambrian 
than in later times. This conclusion receives support from Daly’s 
investigation into the relative abundance of and the time relations of 
the different families of igneous rocks ; * and from his table ° of the 
order of eruption of igneous rocks in different parts of the world 
it is obvious that rocks relatively rich in magnesium were intruded 
more frequently and bulked more largely in pre-Cambrian than in 
Jater periods. 
V. ORIGIN oF THE MAGNESIAN GROUP. 
Whatever cosmic hypothesis be adopted there is good reason to 
suppose that at a very early stage in the earth’s geological history 
what is now known as the “ crust”? underwent a certain amount of 
differentiation leading to a density stratification. Moreover, in view 
of this early differentiation, the crust may be regarded as consisting 
J. P. Iddings, Igneous Rocks, vol. ii, 1913, p. 358. 
G. 8. Rogers, Ann. N.Y. Ac. Sci., xxi, 191], p. 11. 
Watson, Bull. Geol. Soc. Amer., 1915. 
See H. 8. Washington, op. cit. supra. 
5 CO. E. Tilley, ‘‘ The Granite-Gneisses of Southern Eyre Peninsula (South 
Australia) and their Associated Amphibolites”’ : Q.J.G.S8., vol. xxvii, 1921, p. 75. 
6 F, L. Stillwell, “The Metamorphic Rocks of Adelie Land”: Rep. Aust. 
Ant. Hxped., vol. ii, pt. i, 1918. 
7 “The North-Western Highlands of Scotland”: Mem. Geol. Surv. Great 
Britain, 1907. 
8 Igneous Rocks and their Origin, 1914, p. 55. 
9 Op. cit., p. 469. 
10 See Daly, Igneous Rocks and their Origin, 1914, p. 171. 
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