THE OETHOCLASE OK QUARTZ-FREE PORPHYRIES 71 



and a similar roek has been described by Lindgren from the 

 Highwood Mountains in the same state.^ 



(2) THE ORTHOCLASE OR QUARTZ-FREE PORPHYRIES 



Mineral Composition. — The essential constituents are the 

 same as those of syenite. They consist therefore of a compact 

 porphyry gronndmass with porphyritie feldspar (orthoclase) 

 and accessory plagioclase, quartz, mica, hornblende, or minerals 

 of the pyroxene group. More rarely occur zircon, apatite, 

 magnetite, etc., as in the syenites. 



Chemical Composition. — Being poor in quartz, these rocks are 

 a trifle more basic than the quartz porphyries which they other- 

 wise resemble. The following is the composition of an ortho- 

 clase porphyry from Pedazzo as given by Kalkowsld;^ Silica, 

 64.45%; alumina, 16.31%; ferrous oxide, 6.49%; magnesia, 

 0.30% ; lime, 1.10% ; soda, 5.00% ; potash, 5.45% ; water, 0.85%. 



Structure. — Excepting that orthoclase is the porphyritie con- 

 stituent, they are structurally identical with the quartz porphy- 

 ries, and need not be further described here. 



Colors. — These are the same as the quartz porphyries already 

 described. 



Classification and Nomenclature. — The orthoclase or quartz- 

 free porphyries bear the same relation to the syenites as do the 

 quartz porphyries to granite, and the rocks are frequently 

 designated as syenite porphyries. Like the quartz porphyries, 

 they occur in intrusive sheets, dikes, and Java flows associated 

 with the Paleozoic formations. Owing to the frequent absence 

 of accessory minerals of the f erro-magnesia group, the rocks can- 

 not in all cases be classified as are the syenites, and distinctive 

 names based upon other features are often applied. The term 

 orthopJiyr is applied to the normal orthoclase porphyries, and 

 these are subdivided when possible into biotite, hornblende, or 

 augite orthophyr according as either one of these minerals is the 

 predominating accessory. The term rhomhporphyry has been 

 used to designate an orthoclase porphyry found in southern 

 Norway, in which the porphyritie constituent appears in char- 

 acteristic rhombic outlines, and which is further distinguished 

 by a complete absence of quartz and rarity of hornblende. The 



^ Proc. Call. Acad, of Sciences, YoL III, 2(1 series, p. 47. 

 ^ Elemente der Lithologie, p. 86. 



