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IV.—On Professor Dana's Classification of Rocks

Published online by Cambridge University Press:  01 May 2009

T. G. Bonney
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Extract

The two important papers by this accomplished veteran of science, which have appeared in the American Journal of Science (vol. xvi. November and December, 1878) [noticed in this present number of the Geological Magazine pp. 222–225], though, as might be expected, of the highest value, are in one or two respects, as it seems to me, open to question. Professor Dana approaches the subject as a chemical mineralogist: I venture to criticize as a field who checks his conclusions by using the microscope.

Type
Original Articles
Information
Geological Magazine , Volume 6 , Issue 5 , May 1879 , pp. 199 - 203
Copyright
Copyright © Cambridge University Press 1879

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References

page 200 note 1 I have never seen gabbro except under circumstances which suggested deep-seated intrusion; it seems to be the analogue of granite.

page 201 note 1 A System of Mineralogy, pp. 337–361 (ed. 1868).

page 201 note 2 It is no doubt inconvenient that the species microcline has been discovered since the adoption of the term plagioclase; but we may avoid this difficulty by agreeing that the term plagioclase shall be used as a symbol for the group of soda and lime felspars—the character of the other species being so exceptional, and its relations to orthoclase being in most respects close.

page 202 note 1 It is not seldom hard to say (even after chemical analysis) whether a rock should be called a syenite or a diorite, a minette or a kersantite.

page 202 note 2 Analysis of eclogite (garnet and omphacite, with quartz, disthene and mica) from Eppenreuth. Si O2=57·10 Al2 O3=11·66 Fe2 O3=2·84 Fe O=3·22 Mn O=0·31 Mg O=6·37 Ca O=13·80 K2 O=0·81 Na2 O=2·21 H2 O=0·54. Analysis of lherzolite (olivine, enstatite and diopside with picotite) from Kalohelmen. Si O2=37·42 Al2 O3=0·10 Mg 0=48·22 Fe O=8·88 Mn O=0·17 Ni 0=0·23 H2 0=0·71. (Von Lasaulx, Elem. der Petrog.) Analysis of Serpentine (from Cornwall). Si 02 = 38·50 A12 O3 = 1·02 Mg O = 36·40 Ca O = 1·97 Fe2 O3=4·66 Fe O = 3·31 Ni O=0·59 H2 O = 12·35 Fe S = 0·41. Undecomposed residue =1·37. (Q.J.G.S. xxxiii. 925). Analyses of Chlorite schist variable; these are two given by Zirkel (Petrog. i. 311) (1)Si O2=31·54 Al2 O3=5·44 Fe2 O3 = 10·18 Mg O=41·54 H2 O = 9·32 (2) Si O2 = 42·08 Al2 O3 = 3·57 Fe O=26·85 Mn O=0·59 Ca 0 = 1·04 Mg O = 17·10 H2 O = 11·24. In such a grouping, even chemistry, as it seems to me, is fairly thrown overboard.