No. 2.] DANA — SOME POINTS IN LITHOLOGY, 89 



" simplicity " its adoption has been supposed to give to lithological 

 system would be greater if "feldspar" were substituted, and 

 with its present range of constitution, the e"9il would be hardly 

 less. 



7. Rocks diifering mineralogically, and not chemically, like 

 related hornblendic and augitic rocks (the minerals hornblende 

 and augite being dimorphous), are rightly made distinct rocks, 

 since the diflference has depended, to a large extent, on wide- 

 reaching geological operations or conditions, and is, therefore, of 

 great geological significance. 



8. Since quartz is the most widely distributed and therefore 

 the least distinctive of the minerals of rocks, it may rightly be 

 regarded as of subordinate importance in the distinguishing of 

 rocks, and hence not only such names as dioryte and quartz-dio- 

 ryte, trachyte and quartz-trachyte, etc., are acceptable, but also 

 syenyte and quartz-syenyte. 



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 horn- 

 blende rocks ; or those in which hornblende 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 susfo^ests the following additional remark : 

 The incapabilities of the microscope and polariscope have 

 favored the use of the term " plagioclase," and have led some 

 investigators to overlook or slight distinctions in chemical con- 

 stitution. Lithology is to receive hereafter its greatest advances 

 through chemical analyses ; for chemistry alone can clear away 

 the doubts the microscope leaves, and so give that completeness 

 to the Science of Rocks which geology requires for right and 

 comprehensive conclusions. 



Moreover the researches mide in the laboratory to be of real 

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

 tigations 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. 



