224 Notices of Memoirs — Prof. J. D. Dana — On Lithology. 



" 9. Biotite being closely like muscovite in composition, and not 

 less common than it in granites, gneisses and mica schists, and 

 being, moreover, unlike the mineral hornblende in chemical con- 

 stitution and formula, the rocks in which biotite is a chief constituent 

 cannot rightly be put in the same group with hornblende rocks ; or 

 those in which hoi*nblende is a chief constituent in a group of mica- 

 bearing rocks. Consequently the name ' mica-dioryte,' for a rock 

 containing no hornblende, and the name ' hornblende-granite ' for a 

 rock containing no mica but hornblende instead, imply alike false 

 relations. 



" 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 hereafter its greatest advances through chemical 

 analyses ; for chemistry alone can clear away the doubts the micro- 

 scope leaves, and so give that completeness to the Science of Eocks 

 which geology requires for right and comprehensive conclusions. 



" Moreover the researches made in the laboratory to be of real 

 geological value should be, if possible, supplemented by investiga- 

 tions 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 subdivisions in 

 an arrangement 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 constituents, 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 the 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, etc., Felsyte, Trachyte, etc., and the Leucite rock of 

 Wyoming. 



II. The Mica and Soda-Lime Feldspar Series: including Kersantite, 

 Kinzigite; and the nephelitic kinds Miascyte, Litroyte, Phonolite, etc. (These 

 nephelitic kinds belong almost as well in the preceding series.) 



III. The Hornblende and Potash Feldspar Series : including Syenyte 

 (with Quartz-syenyte), Syenyte-gneiss, Hornblende schist, Amphibolyte, Unakyte 



