884 MR JOHN S. FLETT ON 



be subsequently filled by matter infiltrated from the surrounding rocks, which are 

 highly calcareous. 



The large dykes tend to be coarse grained in the centre, where all the ingredients 

 have a short broad form. At the margin, on the other hand, both felspars and ferro- 

 magnesian minerals become very long and narrow, and the rock is very fine grained. 

 The presence of glassy material in the chilled edge of the dykes is very unusual in the 

 camptonites. Occasionally steam cavities of rounded form are to be noted. They are 

 filled in with chlorite and calcite. 



All states of preservation are to be found. Some dykes are so fresh that all the 

 minerals except olivine are practically unchanged. Others, again, have decomposed 

 into a rusty-brown earth, enclosing rounded nodules of less altered material, from which 

 microscopic sections can hardly be prepared. The augite is far more susceptible to 

 weathering than the hornblende, while the biotite, which is probably never entirely 

 absent, though never abundant, is so resistant as to be particularly conspicuous in the 

 more advanced stages of decay. Olivine passes into serpentine and magnetite, and 

 later into calcite and limonite. The augite yields a deep green mineral, which is often 

 chlorite, dusted over with innumerable grains of anatase. The resulting pseudomorph 

 is fibrous, with the fibres parallel to the long axis of the section, and a straight, or 

 nearly straight extinction. The polarisation colours are not uncommonly too high for 

 chlorite, and I suspect that uralite is a very frequent product, especially of the 

 pyroxene of the ground mass, but chloritisation is the usual change in the phenocrysts. 

 Hornblende and biotite pass into chlorite with abundant anatase and occasional rutile. 

 Kaolin and calcite result from the decay of the felspars, and in the more altered rocks 

 rich yellow epidote is often to be seen. That the iron ores are usually titaniferous is 

 evident from the abundance of leucoxene. Calcite is everywhere present, and increases 

 greatly in amount as decomposition advances. 



A peculiar secondary amphibole is found in one or two dykes near Outshore Point 

 in Birsay. The sections show primary hornblende of a deep brown colour in large 

 irregular phenocrysts, with numerous cracks, now filled with calcite and chlorite. Jt 

 resembles that of the dykes previously described (e.g., Scabra Head), and its extinction 

 angle is about 15°. Though much cracked and infiltrated with secondary products, it 

 is mostly quite fresh, but on the outer surface, and lining the fissures, there are 

 occasional spots of a deep blue hornblende. This has a very pronounced cleavage, and 

 may indeed be said to be fibrous. The two minerals are in parallel growth. In one 

 section, which must be nearly parallel to the clinopinakoid, the brown hornblende 

 extinguishes at an obliquity of 15°, the other at 7° on the opposite side of the traces of 

 the prismatic cleavage, and the axis of elasticity most nearly parallel to the prism axis 

 is that of greatest elasticity. The pleochroism is intense. 



C = pale yellow-green, Ij = pale bluish-green, a = deep blue ; absorption, — a>b>C- 



This is, in fact, a secondary mineral of the Riebeckite group, very similar to 

 that which was thoroughly investigated by Cross (X.) in a rock which has much in 



