Professor K. Busz — Granophyre Dyle in Scotland. 437 



beerbachite, after the village Beerbach, where they occur, and so 

 this rock may also be termed beerbachite. 



The felspar, according to its optical orientation, is very nearly 

 a pure anorthite, the maximum extinction angle on the basal 

 plane being between 33° and 34°. It is perfectly clear and 

 colourless, and exhibits no traces of decomposition or any alteration 

 whatever, and only in some parts contains a number of inclusions, 

 which are principally small grains of pyroxene and magnetite, often 

 accumulating in the centre. Pyroxene is present in two varieties, 

 the one being diallage, the other common augite, both about in equal 

 quantities. 



The diallage shows the characteristic structure parallel to the 

 orthopinacoid, and contains numberless thin needle-shaped inclusions 

 of black or brown colour, which are imbedded parallel to the vertical 

 axis as well as to the basal plane, thus producing two intersecting 

 systems of striation ; in some instances they accumulate in the 

 centre, leaving the margin almost free. The colour of the crystals 

 is dull grey, owing to these inclusions. In sections perpendicular 

 to the vertical axis they appear as minute black points, giving the 

 crystals a dust-like appearance. 



The common augite is very pale greyish-green, and almost 

 colourless in thin slices. It contains only very few inclusions, 

 principally grains of magnetite, and can therefore easily be dis- 

 tinguished from diallage. There is no rhombic pyroxene present 

 in this rock. 



The third essential constituent is magnetite, occurring in grains 

 as well as in octahedral crystals, of the same size as the pyroxene 

 and felspar individuals. There are also a few larger patches of 

 quite an irregular form and intergrown with the other constituents. 

 Olivine is rare; it appears in grains, which through decomposition 

 are always partly, and sometimes entirely, altered into green serpen- 

 tine with magnetite dust. There are only very few accessories ; they 

 consist of thin colourless needles of apatite and small light-brown 

 patches of biotite, the latter usually intergrown with augite. 



This gabbro, as mentioned before, does not exhibit the same 

 composition and structure throughout. There exists also a porphyritic 

 variety, which may be termed gabbro-porphyry, or in this case 

 beerbachite-porphyry. This variety, however, was not found in situ ; 

 but numerous xenoliths, to adopt Professor Sollas's term, in the 

 granophyre show this structure. Although having to refer to these 

 xenoliths later on, I will give here a short description of them, 

 in order to show how the porphyritic variety differs from the 

 fine-grained gabbro. The colour of both is the same, the only 

 difference being small black patches as a rule not more than half 

 a centimetre in diameter. They are formed of crystals or crystalline 

 aggregates of triclinic felspar, showing distinctly the cleavage and 

 the twinning lamellas. Their colour being the same as that of the 

 rock, they are not easily discernible at a first glance. 



The ground mass is in many parts of exactly the same composition 

 and appearance as in the fine-grained gabbro, only the felspar 



