No. 2.] DANA — SOME POINTS IN LITUOLOGY. 81 



a potash-bearing rock. The feldspar is a potash-bearing species ; 

 and the mica, whether muscovite or biotite, yields on analysis 

 little less potash than the feldspar, the amount being eight to 

 twelve per cent. These two micas are both present in most 

 granite, gneiss and mica schist: and they are so near akin that 

 they sometimes occur combined in a single crystal — the presence 

 of a little iron in the original material having apparently deter- 

 mined the formation of the latter where it occurs. On the con- 

 trary the hornblende of such rocks contains usually less than one 

 per cent of alkalies, and rarely in any kinds over five per cent. 

 Looking to chemical and mineralogical constitution — the true 

 criterion as to identity among rocks — the strongly drawn line is 

 between the mica-bearing series and the hornblende-bearing series. 

 Granite belongs to a mica and potash-feldspar series ; and syeuyte, 

 whether quartzless or quartz-bearing, to a hornblende and potash- 

 feldspar series. 



Moreover, the original syenyte, from Syene, Egypt (to which 

 the name " syenites" was applied by Pliny and other ancient 

 writers) is a quartz-bearing •' syenites. " The larger part of the 

 syenyte of all Archaean regions is quartz-bearing. The quartz- 

 less kind is seldom met with in Eastern North America, or, as 

 far as explored, in the Rocky Mountain region. There are 

 hornblende granites ; but these are granites which contain horn- 

 blende ill addition to the mica and other inairedients. 



Transitions are common between granite, hornblende-granite 

 and quartz-bearing syenyte ; but they are so also between these 

 and quartzless syenyte, between syenyte-gneiss and ordinary 

 gneiss, between hornblende schist and mica schist, and between 

 these and other rocks. They are throughout lithology a source 

 of difficulty in characterizing kinds of rocks, as already stated. 

 But they do not set aside the fact that the division between the 

 mica and potash-feldspar series and the hornblende and potash- 

 feldspar series is the most reasonable on mineralogical and chemi- 

 cal grounds. 



5. Containing " Plagioclase.'' — The fact that the composition 

 of the triclinic feldspars between the extreme species albite, a 

 sodium aluminum tersilicate, and anorthite, a calcium-alumi- 

 num bisilicate, may be explained by supposing them combina- 

 tions of these species through isomorphous substitutions of the 

 tersilicate and bisilicate (the amount of sodium present deter- 

 mining the amount of tersilicate in the combination, and the 

 Vol. IX. r lHo. 2. 



