348 D. Balsillie — Hypersthene Andesite, Fifeshire. 



The porphyritic felspars, which in many cases are beautifully 

 zoned, belong to the albite-anortliite series and exhibit albite, 

 -carlsbad, and pericline twinning. They are often elongated along 

 the crystal axis a, but are sometimes also equidimensional on the 

 second pinacoid. Examination by Eouque's method shows that in 

 sections normal to the bisectrix there is an angular divergence 

 between the trace of the optic axial plane and (010) of 64°, the 

 corresponding extinction angle on slices perpendicular to 7, measure- 

 ment being made to the (001) cleavages, amounting to from 20° to 

 24°, these values agreeing well with an acid labradorite. Confirma- 

 tion of this identification can be readily obtained from sections that 

 exhibit both carlsbad and albite twinning by the well-known method 

 of Michel Levy, as also from slices cut normally to (010) and (001). 

 The felspar phenocrysts are much more abundant than in the Dumyat 

 rock, are equally fresh and unaltered, and as in its western counter- 

 part the felspars here often contain glass inclusions and show 

 evidence of resorption, though this phenomenon is not nearly so 

 marked as in the rock described by Dr. Fiett. Neither is there here 

 the same indication of flow structure, the phenocrysts in the field of 

 the microscope not showing the rough parallelism that immediately 

 strikes one in the Dumyat rock. As noted, the felspar crystals are 

 often elongated along the crystal axis a, and a single Baveno twin 

 was observed cut almost normally to the bisectrices a, the two 

 halves giving angles of 60° and 61° respectively between the optic 

 axial plane and (010). Each half was in addition twinned on the 

 albite law, and an albite lamella in the one half was seen to be con- 

 tinued across the composition face and be prolonged in the other 

 individual on the pericline law. This optical distinction could easily 

 be made out under high magnification and making use of a thin 

 gypsum plate. 



The pyroxenes include, as has been said, both orthorhombic and 

 monoclinic varieties. The former mineral, which predominates, is 

 optically negative, markedly pleochroic, and may safely, I think, be 

 taken as hypersthene. The crystal outlines are well-marked sections 

 transverse to the crystal axis c, being rectangular in form, witli 

 the prism faces occurring as mere truncations of the corners. Such 

 sections are invariably traversed by a set of irregular cracks, in 

 addition to which there is a well-developed prismatic cleavage. 

 A pinacoidal cleavage I did not observe. The hypersthenes are 

 usually quite fresh and have a tendency to occur as little coteries of 

 small crystals, sometimes in association with the monoclinic pyroxene 

 or in other places along with the felspar's. The pleochroism is as 

 follows: X pale reddish-brown, !F pale yellow, .^ green. 



The monoclinic pyroxene is at once to be distinguished from the 

 hypersthene by its oblique extinctions and higher interference 

 colours. The dark borders that were noted b)'- Dr. Flett to surround 

 the augites in the Dumyat rock do not occur here. The crystal 

 outlines are but ill defined, and twinning occurs on one or more laws 

 that I did not determine. The mineral is not pleochroic, and on the 

 prismatic zone of faces exhibits very often roughly parallel cleavage 

 traces. The angle between the bisectrix Z ( = 7) and (100) was 



