BY H. I. JENSKN. 83 



miarolitic. Each mass has a brownish rim, and a faint yellow 

 interior wliich frequently has a fibrous radial or banded structure. 

 The brownish rim has a double refraction of about 0*020, and a 

 higher refractive index than Canada balsam, hence probably 

 contains a variety of serpentine. The yellow portion is sometimes 

 isotropic, sometimes feebly birefringent in a spherulitic manner, 

 being opticall}'' an aggregate of spherulites. Its refractive index 

 is higher than that of Canada balsam. It consists undoubtedly 

 of chalcedonic silica and opal. That these secondary minerals 

 infill cavities is shown by the fact that the quartz and felspar of 

 the rock always present idiomorphic faces towards them. Included 

 in the chalcedony masses there frequently occur aiinute coin- 

 shaped or hexagonal plates which are perfectly clear, colourless, 

 and almost isotropic, and have a very low refractive index. This 

 mineral is tridymite, and it occurs also in small amount in other 

 parts of the rock. Soda-amphibole amounts to less than 5 %. It 

 is of a dull, slaty-green colour, possesses the characteristic amphi- 

 bole cleavage, and occurs in allotriomorphic corroded crystals 

 which, in general, present straight edges in the cleavage-direc- 

 tion. The extinction angle with the cleavage on the face b(c : c') 

 is 14^ Pleochroism is strong and well marked. On sections 

 showing only one cleavage, the pleochroism may be from blue- 

 black to green, or from green to yellowish-brown. On sections 

 perpendicular to the prism-zone, the colour changes from olive- 

 green to opaque blue-black. As a lies near the c' axis, the 

 absorption-scheme works out to 



c(cleep greenish-blue) >b(olive-green)>a(greenish-yellow). 

 Occasional!}'- small arfvedsonite grains occur together with dusty 

 magnetite in the chalcedony masses. 



Name : Orthophyric Comendite. 



Soil : this rock yields a poor sandy grey soil. 



Silica percentage : the SiOo percentage in this rock was 

 estimated and found to be 76-12. 



Closely allied in composition to the comendites are various 

 rocks belonging to the orthophyres and pantellarites. To these 

 classes belong the rocks now about to be described. 



