of the Buchan District 15 



and satisfactory angular measurements, these results cannot 

 be regarded as being more than approximate. 



The porphyritic mineral, next in importance to the felspars, 

 occurs in almost or quite colourless crystals, whose cross 

 sections show the usual outlines of augite, with a more or less 

 marked prismatic cleavage of nearly 90°. Sections parallel 

 to the axis c show traces of this prismatic cleavage, as also 

 of a second cross cleavage. The angle of extinction in one 

 of these sections I found to be 48° 50'. The terminal planes 

 are not well developed in any of the sections, but in those 

 instances which I could observe at all, suggested the base 

 only. There was not any dichroism. The only inclusions I 

 have observed are a very few crystals or granules of mag- 

 netite. The alterations are wholly to calcite, being easily 

 removed by cold dilute acids. Perhaps half of the crystals, 

 and especially the larger ones, are thus more or less altered. 

 This mineral can scarcely be other than a magnesia-lime augite 

 — that is, diopsid. The augite seems to have been one of the 

 earliest crystallised minerals of this rock, for I find it in- 

 cluded in the plagioclase felspars, and also causing their 

 fracture. Its own often ragged and even cavernous outlines — 

 being surrounded by ground-mass, and this eroded appear- 

 ance not being the result of alteration — strongly suggest to 

 me the action of a molten mass upon pre-existing crystals. 



Associated with the augite is another pyroxenic mineral. 

 It occurs, namely, in prisms of less dimensions than those of 

 the former, and they are distributed through the ground-mass 

 singly or in groups ; and also filling in spaces between other 

 porphyritic crystals. The cross sections of the prism are 

 nearly rectangular. In its freshest condition it is colourless 

 and but faintly fibrous. Its sections are not then dichroic, 

 and it becomes obscured when the prismatic sides are parallel 

 to the plane of polarisation of either of the crossed nicols. 

 Alterations can be traced by the appearance of a fibrous 

 structure, especially conuected with separations across the 

 prism. Its colour then becomes yellowish, and it is dichroic. 



In two instances I found that the ray vibrating parallel to 



i 



c was faintlv red in one case and yellowish brown in the 



i 

 other, while that vibrating perpendicular to c was nearly 

 colourless and green respectively. In one instance, in which 

 the prism was unusually well developed, having terminal 

 planes perpendicular to the sides, the unaltered portions 

 were colourless or faintly brownish yellow ; transverse 



