Williams — Gabbros and JDiorites of the Corilandt Series. 445 



Epidote in Mica-Diorite (No. 28.) 



Twin crys 



source cannot be traced in the alteration of any older constitu- 

 ent. It is generally without 

 terminations but is most 

 remarkable for the peculiar 

 eaten or corroded aspect 

 presented by the crystals, 

 (see fig.) These are often 

 divided into the most com- 

 plicated fret-work of inter- 

 locking tongues. The cleav- 

 age is parallel to the long di- 

 rection of the crystal, and the 

 extinction is parallel to the 

 cleavage lines. The mineral 

 is shown to be epidote and 

 not a pale pyroxene, by the 

 fact that the plane of the 



optic axes is perpendicular to the cleavage lines, 

 tals of this epidote occur as shown in the figure. 



The bright red garnet crystals which are so often found in 

 this rock are most frequent near the edge of its mass and are 

 doubtless an endo-metamorphic product. 



The structure of the mica-diorite is hyp-idiomorphous in 

 the sense of Rosenbusch. It is most closely connected with 

 the diorite proper, into which it grades through the mica- 

 hornblende-diorites as explained above. On the other hand, 

 it passes into the norites through the group of the mica-norites.* 

 Some of these rocks contain much more biotite than hypersthene, 

 closely resembling the hypersthene-bearing mica-diorite from 

 Campo Major in Portugal described by Merianf and the norite 

 facies of the Klausen diorite mass described by Teller and von 

 John.;}: In the latter rock the feldspar has also been shown to 

 belong to the andesine series. 



Quartz-Mica-Diorite. — A quite exceptional member of the 

 massive rocks of the " Cortlandt Series" occurs a short distance 

 eastward of Montrose Station. This has a very light color 

 with only comparatively rare and small flakes of biotite scat- 

 tered through it. It forms a bed of moderate thickness within 

 the dark massive norite against which it is sharply defined, 

 i. e., there is here nothing like a gradual transition from the 

 one rock to the other. 



Professor Dana has described this rock as a granitoid mica- 



*This Journal. Ill, xxxiii, p. 191, March, 1887. 



f Neues Jahrbuch fur Min., etc., Beil. Bd. Ill, p. 292, 1885. 



% Jahrb. k. k. geol. Reichsanst, xxxii, p. 589, 1882. 



