136 The Monoclinic Pyroxenes of New York State. 



Cleavage. A well-developed prismatic cleavage is often pres- 

 ent. Sometimes it is oul}^ apparent in thin sections, but at others 

 is so pronounced as to cause the crystals to cleave easily. An 

 orthopinacoidal cleavage is said hy A. R. Leeds to occur in the 

 pyroxenes of the Adirondack region (E,ef. 34), but it is probably 

 a parting. 



Parting. There exist two pronounced partings due to twin- 

 ning, one parallel to the base, the other to the orthopinacoid. 

 The former is the more abundant and is seen on specimens from 

 nearly every locality. 



Both show themselves on the surface by the existence of numer- 

 ous strise. 



Twinning occurs after two laws, viz., parallel to the base and 

 orthopinacoid, the former being the most common. Usually the 

 only outward evidence of the fact is the striated surface referred 

 to in the preceding paragraph. 



The twinning may be repeated, the alternate twin lamellae being 

 generally extremely thin. At times there is only one twinning 

 plane which' passes through the centre of the cr^^stal, parallel to 

 the orthopinacoid, thus producing twice the number of faces of 

 the same symbol at one end of the individual. 



A curious case of twinning is noted by Prof. Gr. II. Williams 

 on a crystal from Orange Co., in which the lower half of the 

 crystal alone is twinned. (Ref. 60.) 



Perfectly fresh crystals do not generally show twinning. 



Hemihedrism. — According to Prof. Gr. H. Williams this has been 

 shown to occur on crystals from two localities in New York State 

 {Ref. 62), One example is a crystal from Orange Co., which shows 

 planes of different forms grouped about opposite extremities of the 

 vertical axis. From its occurrence on several crystals this inclined 

 hemihedrism seems to be not altogether rare. The Orange Co. 

 crystal shows c (001), s (111), 0(221), e (Oil) andM(lll) at the 

 upper end and c (001) and u (111) at the lower. Another crystal 

 from this same locality shows c (001), s (111,) o (221) and u (111) 

 above, and below the two halves of the crystal in twinning posi- 

 tion and with only o (221) andp (101.) The crystal was carefuUj^ 

 tested by Prof. Williams but showed no pyro-electricity. A 

 second instance of hemihedrism is noted in a crystal from Grassj^ 

 Lake, St. Lawrence Co., with M (111,) s (111), o (221) at one end 

 and^(r01),c (001), w (HI), 8(0.1) and o (221) at the other. 



