311 
not be summarized under any simple rule. In this connection 
it may suffice to make mention of the classical researches which 
have been made by Ippines in the western States of America, 
by A. Gewie in the British Isles, and by Bröccer in the Chri- 
stiania district’. 
Most authors agree, as is well known to every petro- 
logist, that, wherever a regularity in the sequence of igneous 
rocks has been observed or inferred, the cause of the regularity 
must be sought for in the laws which control the processes 
of differentiation by which the individual rocks of the particular 
petrographical province are believed tu have descended from a 
common parent magma. From the order of succession of the 
rocks conciusions, accordingly, are drawn as to the nature of 
the suppused processes of differentiation. From this point of 
view the different rules of chronological sequence are out of 
harmony, and various attempts, which in the opinion of the 
present writer are not quite satisfactory, have been made to 
reconcile them”. 
Harker® points out that when only large Plutonic masses 
are concerned the ‘rule of decreasing basicity’ is of world wide 
application, and this distinction is no doubt of great value in 
the ‘treatment of the problem. The ‘rule of decreasing basicity’ 
may as in the Christiania district hold good in some cases also 
for extrusive rocks, but as a general law (with some exceptions) 
it is only applicable to abyssal rocks which belong to the same 
cycle of igneous activity. With this limitation the ‘rule of 
1 J. P. Ippınss, The Origin of Igneous Rocks. Bull. Phil. Soc. Washington, 
vol. 12, 1892. — Idem, Journal of Geology, vol. I, 1893, р. 840. — Idem, 
Quart. Journ. Geol. Soc., vol. 53, 1896, р. 617. — А. GEIKIE, ibidem, vol. 
52, 1896, Proceedings р. 178. — W. С. Ввбасев, Op. cit. — A. Harker, 
Natural History of Igneous Rocks 1909, pp. 110—146. 
2 W. С. BROGGER, Eruptivgesteine des Kristianiagebietes Ш, 1898, р. 
363. — A. Harker, Natural History of Igneous Rocks, 1909, pp. 112 
—117. 
Op. cit. p. 114. — Tertiary Igneous Rocks of Skye, 1904, p. 421. 
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