98 POPULAR SCIENCE REVIEW. 
The discussion suggests the following additional remark : 
The incapabilities of the microscope and polariscope have favoured the use 
of the term “ plagioclase,” and have led some investigators to overlook or 
slight distinctions in chemical constitution. Lithology is to receive here- 
after its greatest advances through chemical analysis ; for chemistry alone 
can clear away the doubts the microscope leaves, and so give that complete- 
ness to the Science of Rocks which geology requires for right and compre- 
hensive conclusions. 
Moreover the researches made in the laboratory, to be of real geological 
value should be, if possible, supplemented by investigations in the field as to 
transitions among the rocks, and as to other kinds of relations. This field 
work has often been well done, but not so by all lithological investigators. 
The principles presented lead to the following sub-divisions in an arrange- 
ment of crystalline rocks, exclusive of the Calcareous and Quartzose kinds. 
Since leucite is a potash-alumina silicate, like orthoclase and microcline (it 
affording twenty per cent, or more of potash), it is here referred to the same 
group with the potash feldspars, and nephelite, sodalite and the saussurites 
being eminently soda-bearing species, they are included with the soda-lime 
feldspars (anorthite to albite.) This reference for lithological purposes of 
these minerals is sustained by their resemblance to the feldspars in consti- 
tuents, and also in the quantivalent ratios between the alkalies, alumina and 
silica, this ratio being in leucite 1 3 8, as in andesite, and in sodalite and 
nephelite 1 3 4, as in anorthite. The term potash-feldspar, as used in the 
headings below, is hence to be understood as covering orthoclase, microcline 
and leucite ; and soda-lime feldspar , as including the triclinic feldspars from 
anorthite to albite, and also nephelite, sodalite and saussurites. 
The arrangement is as follows : — In the first series, the rocks graduate 
into kinds which are all feldspar, and into others that are all mica, and yet 
the amount of potash present is approximately the same. 
I. The Mica and Potash Feldspar Series : including Granite, Granulyte, 
Gneiss, Protogine, Mica schist, &c., Felsyte, Trachyte, &c., and the Leucite 
rock of Wyoming. 
II. The Mica and Soda-Lime Feldspar Series : including Kersantite, Kinzi- 
gite, and the Nephelitic kinds, Miascyte, Ditroyt, Phonolyte, &c. (These 
nephelitic kinds belong almost as well in the preceding series.) 
III. The Hornblende and Potash-Feldspar Series : including Syeny te (with 
Quartz-syenyte), Syenyte-gneiss, Hornblende schist, Amphibolyte, Unakyte 
(this last containing epidote in place of hornblende), and the Nephelitic 
species Zircon-Syenyte, Foyayte. 
IV. The Hornblende and Soda-Lime Feldspar Sei'ies : including Dioryte 
(with Propylyte), Andesyte, Labradioryte (or Labradcr-dioryte), etc., and 
the saussurite rock, Euphotide. 
V. The Pyroxene and Potash Feldspar Series : including Amphigenyte. 
VI. The Pyroxene and Soda-Lime Feldspar Series: including Augile, 
Andesyte, Noryte (Ilypersthenyte .and Gabbro in part), Hypersthenyte 
(containing true Hypersthene), Doleryte (comprising Basalt and Diabase), 
Nephelinyte, etc. 
VII. Pyroxene , Garnet, Epidote and Chrysolite Pocks, containing little or 
no Feldspar: including Pyroxenyte, Lherzolyte, Garnetyte (Garnet rock), 
Eclogyte, Epidosyte, Chrysolyte or Dunyte (Chrysolite rock), &c. 
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