THE AROH^AX FORMATION OF THE AEUKUMA PLATEAU. 919 



foiHifl, being often cut sharply hy wide dykes of tlie hiotite-amnite in 

 a most confused manner, as is represented in Fif»". 1, l^Jate XX TV. 

 Therefore the biotite-gTanite must he of later age than the hornhlende- 

 bearing variety. When sneli dykes of the younger granite appear 

 repeatedly within a short distance it is often not easy to say wliich is 

 the foreign mass and which the intruded (^ne. It is not necessary 

 to point out the localities of snch oceurrences, as these perplexing 

 cases are inevitablv met witli durinii- field-work. 



A. Amphibole-Granites and Biolite-Graniles. 



The dominant rock of tlie region, the .aniphibole-granite, has, as is 

 already stated, a very uniform composition which varies only thrt^ugh 

 local fluctuations in (piantit^y of the several ingredients. AM the rocks 

 included under the ]n'esent group contain, therefore, quartz, orthoelasr, 

 inin'ocline, vJdfu'nda.^e, aiiiphiJtoh'^ hiotife, titanitt', apatite, zircon, and 

 opaque iron ores. To tliese may be added the usual, secondary 

 accessories, such as rpidote, chlorite, mu^covite. and iron-filanci'. 



!)ef()]-e we enter into the detailed description of the individual 

 occurrences of granites, it may be well to speak of the minerals which 

 enter into the composition of the rocks. Although their properties 

 do not ditfer in essential points from those of other granites, still it 

 seems well to give here some consideration to ~them as likely to 

 prove of some use for the mapping geologists of onr Geological Survey. 



The feldspars are of three varieties, viz., orthoclase, microcline, 

 and plagioclase. The secondary muscovite, resulting from decomposi- 

 tion, is common to all of them ; while epidote is very rarely observed, 

 indicating that the plagioclase is of an acid nature, and consequently 

 poor in lime. In regard to dimensions, there occur large and 

 small individuîds which, althouofh belong-in"- to the same o-eneration, 



