192 Canadian Record of Science. 



specimens is decomposed. The product of decomposition 

 consists sometimes of a finely granular mixture of chlorite, 

 and a rhombohedral carbonate with occasional quartz 

 grains between them, the whole constituting a grey almost 

 opaque mass. In other specimens the augite is changed 

 into a yellowish bastite which then fills up not only the 

 space originally occupied by the augite but also penetrates 

 into the small fissures of the rock and forms thread-like 

 veins and scales even in the feldspar grains. In other 

 specimens it is converted into a mineral resembling serpen- 

 tine. When both pyroxenes occur near one another in the 

 rock, the angite is generally intimately intermingled with 

 the rhombic pyroxene. 



Ehombic Pyroxene (Hypersthene). — This mineral, 

 which occurs so often with augite, does not essentially 

 differ from the latter as far as can be ascertained from its 

 thin sections either in index of refraction, in double refrac- 

 tion or in color. It is however strongly pleochroic with 

 the following colors : 



a=red, fa= yellowish green, c= green. 



The absorption is a > fa > jC, the difference between a 

 and fa being very small. 



Its rhombic character was determined by the following, 

 observations in the case of a hand-specimen from the 

 Township of Chilton in which the mineral occurred in fresh 

 condition and in larger quantity than usual. Sections 

 parallel to the base showed the two cleavages of the prism 

 which intersect almost at right angles, as well as a third 

 more perfect set of cleavages to which small black rods are 

 often parallel. Since the direction of the extinction is also 

 parallel to this latter cleavage it must be in the direction of 

 a pinacoid. In convergent light there is seen on the 

 basal section a bisectrix but not an optic axis as in the 

 case of a monoclinic pyroxene. When a section in which 

 an optic axis appears is examined, the above mentioned 

 pinacoidal cleavage is found to be parallel to the plane of 

 the optic axes. The pinacoid in question is therefore 



