354 XIEr/T.-GEN. C. A. M c MAHON ON ROCKS OF [Aug. 1 894, 



No. 41, both macro- and microscopically considered, is seen to be 

 made up of two distinct layers. Under the microscope the purplish 

 layer is seen to be almost wholly composed of fine granular augite, 

 with a small proportion of the reddish buff-coloured felspathic base. 

 The second, and greenish-coloured layer is made up of minute, 

 slightly-elongated and irregularly-shaped grains of felspathic material 

 that has a distinctly lamellar structure. This is dappled over with 

 a granular, translucent to opaque substance, for which I have no 

 name. Between these two layers the augite has passed into dichroic, 

 finely granular hornblende. 



jSTo. 42 is essentially the same rock as the green portion of No. 41. 

 The felspathic base is predominant and the pyroxene subordinate ; 

 hence the comparatively low specific gravity (2'65) of this specimen. 

 The rock is composed of the felspathic base (in which a lamellar 

 arrangement of the materials is apparent), with greenish hornblende, 

 a little augite, some ferrite and opacite, a little sphene, and a little 

 secondary quartz associated with the hornblende. The hornblende 

 exhibits a linear arrangement of its crystals and granules parallel 

 to the lamellar structure of the base, and is apparently due to the 

 action of heated water acting along these lines of lamination. 



No. 45 is essentially the same rock as the last. The lamellar 

 arrangement above alluded to is not apparent in the base, and con- 

 sequently this structure is not seen in the hornblende ; but the 

 study of this specimen leaves no doubt of the fact that the formation 

 of this secondary hornblende is connected with the action of heated 

 aqueous agencies, for one side of the hand-specimen (originally, I 

 presume, the wall of a crack) is covered with hornblende, and 

 numerous microscopic cracks, lined with this mineral and leading 

 up to and traversing hornblende-crystals, may be seen in the slice. 

 These crystals are in the form of more or less regular rhombs (the 

 prismatic angle of one of them is 126°) ; internally they are made 

 up of minute granular patches of hornblende, which here and there 

 coalesce to form homogeneous platy crystals, exhibiting a single 

 cleavage from which the angle of extinction measures 0° to 12°, 

 with an average of 8°. Dichroism is distinct, though somewhat 

 feeble. Extinction in the small patches is not always uniform 

 throughout the rhomb, but this may be due to imperfect twinning. 



This slice affords a very instructive example of the mode in which 

 much of the secondary hornblende in these rocks owes its birth to 

 active aqueous agencies permeating the pores of the rock. 



No. 44 is a highly altered rock. It is composed principally of 

 what appears to have once been a felspathic base and actinolite. 

 In some parts of the slice there is much of what is probably zoisite ; 

 it is colourless, and its double refraction is so low as to be practically 

 nil. One of my slices of this rock contains an abundance of sphene 

 in grains and some iron, probably ilmenite. 



The actinolite is of a pale green colour and occurs generally in 



