S. Weidman — Amphibole Hudsonite. 229 



1853. This was long before the microscope came into use. and 

 hence nothing was known concerning its optical properties. 



Description of the Amphibole Hudsonite. — The macroscopic 

 appearance of the hudsonite closely agrees with the original 

 description by Beck. Its color is black to brownish black. 

 Luster, vitreous. Besides the very perfect prismatic cleavage 

 of 54° to 56°, * there is a well developed transverse parting with 

 distinct twinning lamellae parallel to the parting, which is 

 identical in character and position with that described by 

 G. H. Williamsf in actinolite. This parting or pseudo-cleavage 

 is a marked feature of the hudsonite. On the orthopinacoid 

 it forms striations normal to the prismatic cleavage, and on the 

 clinopinacoid the striations make an angle of about 76° with 

 the prismatic cleavage. These structure planes are common in 

 pyroxene, where they are considered to be in the plane of the 

 basal pinacoid. It was very likely this pronounced parting 

 which led Beck to liken this mineral to pyroxene in his original 

 description. 



Under the microscope the mineral is seen to be free from 

 alteration, but it contains a few small areas of limpid quartz 

 and a very slight infiltration of extraneous material in the older 

 fractures. Following Tschermak and Dana, the plane of part- 

 ing and of twinning lamellae above noted is considered the 

 basal pinacoid (001) in order to make it correspond with the 

 similar structural plane in pyroxene, instead of considering it 

 in the plane of the orthodome (101) in accordance with other 

 text-books. A thin section which was cut as near as possible 

 in the plane of the clinopinacoid showed the transverse part- 

 ing planes to form an angle of about 76° with the prismatic 

 cleavage, and thus the slide was cut sufficiently near the plane 

 of the clinopinacoid to furnish the extinction angle of the hud- 

 sonite. A series of measurements showed the plane of extinc- 

 tion to be about 9° from the vertical axis in the obtuse 

 axial angle /3. This axis of elasticity, being apparently less 

 than the one normal to it in this plane, it is considered the 

 c axis, and hence c /\ c = + 9°. The optic angle and charac- 

 ter could not be satisfactorily determined. The pleochroism 

 is pronounced and absorption strong: a = light olive green, 

 6 = green with slight tinge of yellow, c = green with slight 

 tinge of blue. Absorption, c > b > a. 



While transverse parting in actinolite and common green horn- 

 blende parallel to the base (001) has been observed J by a number 

 of writers, twinning lamellae parallel to this plane apparently 



*Prof. Penfield has since measured the angle of the prismatic cleavage 

 with a reflection goniometer and found the values m a. m — 55° 31'. 

 - f This Journal, xxxix, 352, 1890. 

 ^ {See Zirkel, 2d ed., i, 300. 



