THE TKAP DYKES OF THE ORKNEYS. 893 



rays, but especially for fr and c, which are dark brown. The optic axial angle is very 

 small. Twinning on Tschermak's law was not made out, and the extinction may have 

 an obliquity of 5°. The biotite encloses olivine, augite, magnetite, perofskite, apatite. 

 In weathering it passes into a deep green chlorite, but this change is confined to the 

 margin of certain sections (see PL III. fig. 4). 



Augite is very abundant in small crystals of the second generation. The prisms are 

 elongated, and average "015 mm. in breadth by '1 mm. in length. They are very 

 idiomorphic, and in transverse section have usually six sides, the clinopinakoid not being 

 developed. They are irregularly scattered, and often in radiating groups of six or 

 seven. Their colour is purplish-brown, resembling the outer zone of the phenocrysts. 

 Owing to their small size, they are almost without cleavage, but it is usually to be traced 

 in the longitudinal sections. Their extinction angle ranges up to 43°. They are not 

 dichroic, and very commonly show hour-glass structure and twinning similar to that of 

 the phenocrysts. These little augites are enclosed in the periphery of the biotite plates, 

 but are never seen in the centre, and very frequently the edges of the biotite are 

 moulded on them. In that case the dark brown rim is absent. In the groundmass 

 biotite in small, sometimes sharply-formed crystals is mingled with the augite, and 

 adherent to its margins or surrounding magnetite. On the surfaces of the olivines 

 biotite and augite are usually implanted. 



Melilite in some of the slides and in some parts of them is quite common, in others 

 it is not recognisable owing to decomposition. It is best found fresh on the edges of 

 the biotite. Here it has rather perfect form, squarish rectangular or lath shaped. It is 

 colourless, not dichroic, with a cleavage parallel to the length of the section very well 

 defined, and another less frequently seen at right angles to this. It contains glass 

 inclusions, but regular peg-structure is not seen. The extinction is straight, and the 

 polarisation colours are pale grey. Transverse sections are not recognisable. Parallel 

 growths between biotite and melilite, as figured by Berwerth (XVII. , pi. 10), were 

 not found. Care must be taken to distinguish it from the abundant apatite, which 

 approaches it in size, but the lower refractive index, the more perfect cleavage, the 

 sharply rectangular outlines, and the frequent decomposition, assist in identifying the 

 melilite. The apatite penetrates all the ingredients, but the melilite is moulded on the 

 others, except biotite occasionally. It is optically negative. 



In some parts of the slides a clear isotropic groundmass resembling glassy matter is 

 frequently observed, but usually it is turbid and granular, and in polarised light breaks 

 up into a mosaic of angular grains of calcite with fibrous zeolites, and possibly analcite. 

 This material shows mostly no cleavage or trace of regular structure, but at times it 

 encloses little rectangular prisms which are apparently melilite mostly decomposed 

 into zeolites, which are fibrous, and extinguish parallel to the prism axis, while weakly 

 polarising grains of still fresh material may be found in the centre. Whether there has 

 been originally a glassy residuum or a final crystallisation of nepheline, as Adams 

 suggests (XVIII. , p. 278), cannot now be made out. 



