and their Contact Zones. 39 



and these colours are somewhat lighter than is usual in the 

 hornblende of the district. In those slices which are 

 parallel to the most perfect cleavage there are only faint 

 traces of cross cleavage; in those perpendicular to this 

 direction the cleavage in one direction is well marked ; less 

 well marked are irregular directions crossing it. More rarely 

 I have met with sections which showed a very imperfect 

 prismatic cleavage, approximately near to 124° 30'. The 

 angles of inclination of the principal optical section against 

 the direction of the most marked cleavage (i.e.. ao-ainst the 

 direction of the axis c), are higher than is usual in horn- 

 blende. My measurements gave me, for those sections 

 showing a well-marked cleavage, 12° — 22°, and in one 

 instance 26°. The sections of this mineral are moderately, 

 and even occasionally strongly, dichroic, in spite of the 

 rather light tints of the mineral. 



These observations indicate a mono clinic mineral, occur- 

 ring in irregularly bounded crystals, having a perfect cleav- 

 age parallel to ii, with a metalloid and vitreous lustre, a 

 less perfect cleavage parallel to I, and an imperfect cleav- 

 age parallel to ii. Its monoclinic optical characters, its 

 polychroism, and its prismatic cleavage near 124° 30', refer 

 it to amphibole, while its physical character and its marked 



cleavage in the direction ii point to anthophyllite. I there- 

 fore refer it to that mineral sub-species which Des Cloizeaux 

 has termed amphibole-anthophyllite. I have observed an 

 interesting parallel between this mineral and the pyroxenic 

 mineral diallage. It may be said that this amphibole- 

 anthophyllite stands in the same relation to amphibole that 

 diallage does to augite.* The inclusions in this mineral 

 are peculiar. They may be regarded as constituents of the 

 rock of which amphibole-anthophyllite forms almost the 

 whole ; but it will be most convenient to deal with them in 

 this place. 



Orthorhombic Pyroxene — A mineral which I refer to 

 this group is in imperfect crystals, or in masses of crystalline 

 granules. It is found between the cleavable masses of 

 amphibole rather than in them, and conforms to their outlines. 

 Macroscopically, I have observed it to be of a rather light clear 

 brown colour, having a resinous lustre and a marked cleavage, 



* Anthophyllite is to amphibole as enstatite is to augite. Eosenbusch 

 MikrosJtojnsche Physiograjphie der Massigen Gesteine, p. 263. 



