Notices of Memoirs — Prof, J. D. Dana — On Lithology. 223 



" 2. In determining the rocks to be grouped as one in hind under 

 a common name, near identity in the chemical and mineral composi- 

 tion of the chief constituents is the main point to be considered ; not 

 near identity in their crystalline forms, for isomorphism presupposes 

 diversity of composition. 



" 3. Distinction of kind should be based on difference in chemical 

 and mineral constitution as regards the chief constituents. When 

 such difference exists, rocks are different in kind, and need, for the 

 purposes of geology, distinct names. If it does not exist, the dis- 

 tinction is only that of variety ; unless (as in the case of trachyte and 

 felsyte), the very wide extension of the rock under persistent 

 characters makes a distinction of name important to geology. 



" 4. It follows from the preceding, that differences in texture : as 

 coarse, or fine, or aphanitic ; porphyritic, or non-porphyritic ; stoney 

 throughout, or having uniiidividualized portions among the stoney 

 grains ; and differences in microscopic inclusions ; are no basis for 

 a distinction of kind among rocks, but only of variety ; and that 

 porphyritic structure is of hardly more consequence than coarse or 

 fine granular. 



" 5. No marked change in the constituents of the earth's erupted 

 material occurred after the close of the Cretaceous period, or just 

 before the commencement of the Tertiary era ; and, hence, no ground 

 exists for the distinction of ' older ' and ' younger ' among eruptive 

 rocks. The ' younger ' eruptive rocks are essentially like the ' older ' 

 in chemical composition and their chief mineral constituents ; and 

 they differ when at all only in texture and some other points of as 

 little importance — qualities that distinguish merely varieties, and 

 which have proceeded from greater prevalence in these later times 

 of subaerial eruptions. 



" 6. Since ' plagioclase ' is not the name of a mineral species, — 

 several minerals, of widely different compositions, being embraced 

 under it — it is a confounding of differences and resemblances to 

 speak of it as a constituent of a rock. And since it now includes, 

 through the defining of the feldspar microcline, a large part of 

 potash feldspar, which had been supposed to be orthoclase, it has 

 become almost synonymous with the term feldspar. The ' 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 evil would be hardly less. 



" 7. Eocks differing mineralogically, and not chemically, like re- 

 lated hornblendic and augitic rocks (the minerals hornblende and 

 augite being dimorphous), are rightly made distinct rocks, since the 

 difference has depended, to a large extent, on wide-reaching geolo- 

 gical 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-dioryte, trachyte and quartz- 

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



