228 BULLETIN OF THE 



nion ; in some specimens the rock is dotted with small white crystals of 

 sodalite. In one instance the rock holds large phenocrysts of flesh- 

 colored feldspar in tabular prisms with tei'minal planes, and is porous 

 owing to the presence of small irregular cavities, whose rough walls are 

 lined witli limonite. Plates of Liotite are rarely seen. There is a dis- 

 tinct fluidal structure in the dikes and sheets which is visible in the 

 hand specimens by the parallel aiTangement of the phenocrysts and a 

 slight fissility parallel to the plane of flow. 



Characters in. Thin Sections. — Feldsjmr. — In the thin sections of this 

 type the feldsjiar phenocrysts appear in long rectangular sections, or in 

 broader and less regular crystals. Carlsbad twins are as frequent as 

 single crystals. A certain number of these feldsjiars polarize homoge- 

 neously, and extinguish parallel to the length of the rectangular sec- 

 tions, or nearly so, like sanidine ; but in many sections a very fine 

 multiple twinning is present parallel to the brachj^pinacoid (albite law), 

 and sometimes there is present with this a faint double twinning (mi- 

 crocline structure), or segments are multiply twinned in one direction, 

 whicli is not the same in adjoining segments of the feldspar, ("leavage 

 sections were jirepared from one of tliese phenocrysts with the following 

 result : — 



Basal cleavage (0 P) extinction 2i° to 3° oblique to the trace of the 



2d cleavage (qq P ^ ). 

 Second cleavage (oo 1' oo ) extinction about 9° oblique to the trace of 



the basal cleavage. 

 Specific gravity of the piece 2.583. 



This specimen is therefore a triclinic feldspar intermediate between 

 microcline and albite (soda-rnicrocline — anorthoclase) and the anomalous 

 polarization of the feldspar sections in the rock is thus explained. These 

 phenocrysts contain as inclusions apatite, sodalite, augite, fcgirine, and 

 biotite. The pegirine needles appear to have formed toward the close 

 of the feldspar period, as they are commonly included in the peripheral 

 portion of the crystal. 



Sodalite is present in some specimens, occurring either in crj'stals 

 large enough to dot the rock macroscopically, or only visible in tlie 

 section. It appears in tlie slides in the usual apparent hexagons or 

 squares, sometimes rounded or corroded by the groundmass. The min- 

 eral is colorless, with a low refractive index and isotropic, unless zeo- 

 litization has occiu'red. It sometimes contains secondary calcite. Tlie 

 mineral gelatinizes easily witli acid, and the micro-chemical tests sliow 



