THE TRAP DYKES OF THE ORKNEYS. 895 



mid dyke and east dyke the ingredients are the same, but the biotite is less abundant, 

 and in smaller crystals which are deep brown, and adhere to olivine, augite, and 

 magnetite. The east dyke is much decomposed, and yielded no evidence of melilite. 

 The mid dyke is fresher, and in the hand specimen has a finely nodular appearance. 

 The little dark brown biotites are often idiomorphic, and distinct traces of melilite were 

 observed in crystals irregular in shape or approaching rectangular, moulded on the augite, 

 which plays the chief part in the groundmass, and undergoing the same decomposition 

 as in the other slides. 



In Naversdale, Orphir, a dyke is seen cutting the more northerly burn in the valley 

 about half-a-mile above the sheepfold. It is 3 feet broad, and in the hand specimen 

 the mica is very evident in little glittering scales, but there is no marked parallel 

 structure ; there is much calcite in rounded spots. 



Under the microscope it is very similar to the east dyke at Rennibuster point (PL III. 

 fig. 5). It contains many olivines, weathered to green and yellow serpentine and mag- 

 netite, and surrounded by a fringe of biotite and little augites. Large augites with similar 

 appearance and properties to those in the Rennibuster dyke accompany the olivine. 

 The biotite occurs also in large, irregular plates, with similar enclosures, but here 

 without a darker rim ; and the little scales in the groundmass are absent. The 

 groundmass consists of little idiomorphic augite crystals scattered through what appears 

 under low powers a turbid, granular material, but on higher magnification resolves 

 itself into melilite in irregularly-shaped intersertal masses, which fill up the interstices 

 between the other minerals. Over fairly large areas the melilite has the same cleavage 

 and extinction, though penetrated by many augites. The structure is exactly that of 

 the ophitic dolerites, where augite encloses idiomorphic plagioclase. Peg-structure is 

 here very perfect, and perpendicular to it a rather ill-defined cleavage. The mineral is 

 never absolutely fresh, but always granular, turbid, brownish in ordinary light ; it is 

 not dichroic. In polarised light decomposition is seen to have followed the direction of 

 the pegs and of the cleavage to a less extent, with the production of a fine fibrous 

 secondary product, which at first forms a rectangular network over the mineral, and, as 

 the change proceeds, extends gradually over the meshes. The fibres are mostly parallel, 

 and although the polarisation colours are too high for melilite, the aggregate has a 

 straight extinction. Many fibres, however, have an indefinite orientation, so that, when 

 the section is at its darkest, extinction is never absolute, but bright flakes are to be seen. 

 These are mostly at the edges and in the cleavage cracks. In other sections the fibres 

 have an entirely irregular or radiate arrangement ; these may be pseudomorphs after 

 melilite in transverse section. Recognisable calcite does not appear till comparatively 

 late in the process. Analcite, calcite, and other minerals of secondary origin, which are 

 undeterminable, occupy other parts of the sections. An isotropic mineral of irregular 

 corroded form, full of dark inclusions, and resembling nosean, is present in very small 

 quantity. Apatite and perofskite are both abundant. 



The sequence of crystallisation appears to have been apatite, perofskite, magnetite ; 



