THE HOUNIJLENDE SYENITK. 177 



Tlu' primary teldspars are clouded witli alteration products, while the 

 secoudar}- ones are (dear. The j)riniary and the secondar\' minerals bear 

 the same relations to one another as they do in the granites. Tlie horn- 

 blende is in dark brownisli-o-reen crystals that are idiomorphic in the pris- 

 matic zone, l)ut ))adly terminated at their extremities. It is nearly always 

 more or less com])letely altered to chlorite. The sphene is also rarely IVl'sIi. 

 It is usually changed into a cloudy, light-colored substance that looks yellow 

 in r(itlected light. In general appearance it resembles the lencoxene so 

 often seen surrounding ilmenite or titaniferous magnetite in greenstones, 

 and hence it is regarded as this substance. 



A similar alteration' of sphene into leucoxene has been described by 

 Weryeke, -s^on Kuch, Velani, and Grroth, the latter author regarding it as a 

 product of weathering. In the Michigan rock the leucoxene forms ])erfect 

 jjseudomorphs, which retain the diamond-shaped cross-section of the origi- 

 nal spliene. Whether it is a product of weathering or a result of dynamic 

 metamorphism can not be told. 



The quartz, which is always present to some extent, but neyer so 

 abundantly as in the granites, occurs sometimes as small grains with an 

 undidatory extinction, sometimes as larger ones broken up into an aggregate 

 of differently orientated particles Most of the mineral, however, is in the 

 angular spaces between the feldspars or in the cracks trayersing the older 

 constituents. A small portion of the quartz may be original, but the greater 

 portion is thought to be secondary. 



The structure of the gneissoid syenites is identical with that of the 

 granites; so it iieeds no discussion in this place. The syenite, as well as 

 the granite, is an igneous rock that has suffered dynamic metamori)liism. 

 The latter is a (^uartz-biotite-orthoclase rock, and the syenite an aggregate 

 of hornblende and orthoclase. Even were the two rocks not distinguished 

 by the abundance of quartz in the one and its rarity in the other, they 

 would be distinguished by the })resence of the biotite in the granite and 

 of the hornblende in the syenite. 



' Lehrbucli der Petrographie, by F. Zirkel, Vol. I, 1893, p. 410. 

 MON XXVIII 1-' 



