THE TRAP DYKES OF THE ORKNEYS. 905 



Fig. 2. Camptonite, East Dyke of Hoxa, South Ronaldshay, Orkney — magnified 20 diameters. The 

 nature of the groundmass is better seen than in the previous figure. Some of the clear spots are analcite in 

 irregular grains. Pale augite phenocrysts are few. The elongated section of hornblende shows an internal 

 dark laminated nucleus and a clearer peripheral zone. On the right is a phenocryst with graphic augite-horn- 

 blende centre, dark with magnetite, and two external zones of hornblende. In the centre and on the left are 

 two similar crystals, but in the central one the sides are augite, the ends hornblende ; the other is completely 

 surrounded by a narrow border of augite. 



Fig. 3. Camptonite, "West Dyke of Hoxa, South Ronaldshay, Orkney — magnified 20 diameters. In the 

 centre an "analcite phenocryst." The clear space on the left is due to the disappearance of a hornblende 

 crystal. Above the centre a clear augite, to the right a zonal somewhat irregular hornblende. Below and to 

 the left several graphic intergrowths with hornblende borders. 



Fig. 4. Camptonite, West Dyke of Hoxa, South Ronaldshay, Orkney — magnified 20 diameters. On 

 the upper margin, part of one of the pale ocelli, with long, prismatic, hornblende crystals, and clear 

 felspars, embedded in a brownish, granular, somewhat decomposed glass. Below the centre a large 

 phenocryst of dark brown laminated hornblende intensely corroded, surrounded by a thin, clear hornblende 

 zone, and around this a narrow augite-hornblende intergrowth. The external border is partly augite, partly 

 hornblende. To the left of this a phenocryst with a centre of graphic augite-hornblende intergrowth, dark 

 with magnetite, and a border of hornblende on the upper end, of augite elsewhere. 



Fig. 5. Fourchite, section from a loose block of trap found on the shore of Kirkwall Bay near Quanter- 

 ness Skerry — magnified 40 diameters. In the centre a large phenocryst, which consists of a graphic inter- 

 growth of augite and hornblende, with comparatively little magnetite, and mixed with a considerable quantity 

 of a clear transparent glass. The upper margin is augite, the lower hornblende. The crystal is repeatedly 

 twinned and the twin planes pass transversely through the whole structure. The groundmass contains mag- 

 netite and hornblende embedded in a clear transparent glass which is full of dendritic skeleton crystals of 

 hornblende. 



Fig. 6. Section of the same rock as No. 5 — magnified 20 diameters. Showing a large, much corroded 

 phenocryst of dark laminated hornblende, with enclosures of fine magnetite dust in lines parallel to the faces 

 of the prism (110). It is surrounded by a thin margin of clear hornblende. 



Plate III. 



Fig. 1. Augite Camptonite, section from a loose block of trap found on the beach near Burness, Firth, 

 Orkney — magnified 20 diameters. Pale violet augite in small rounded crystals, which are often perfectly 

 idiomorphic, surrounded by plagioclase felspar, mixed with magnetite. 



Fig. 2. Diabase, Binniaro, Firth, Orkney — magnified 20 diameters. Phenocrysts of olivine altered into 

 serpentine (above) and of augite in a glomero-porphyritic aggregate (below). The groundmass consists of 

 idiomorphic lath-shaped felspar, which is included in chloritised augite in an ophitic fashion. 



Fig. 3. Monchiquite, Grainbank, Kirkwall, Orkney — magnified 20 diameters. Phenocrysts of olivine, 

 altered into serpentine, and of augite, lying in a groundmass of augite, biotite, and magnetite, with a small 

 quantity of a granular glassy base. 



Fig. 4. Alnoite, Rennibuster Point (east dyke), near Kirkwall, Orkney — magnified 20 diameters. In 

 the upper part of the figure a phenocryst of augite with a pale centre and dark violet-brown margin. Biotite 

 is seen in large irregular plates below, and on the right cut perpendicular to the cleavage ; on the left a plate 

 of biotite cut parallel to the cleavage. The biotite encloses magnetite, apatite, and at its margins the small 

 augites of the groundmass. A pseudomorph of calcite after olivine, enclosing perofskite on the left below. 

 Details of the groundmass are not shown. 



Fig. 5. Alnoite, Naversdale, Orphir, Orkney — magnified 20 diameters. The left of the figure shows 

 one or two large plates of biotite cut parallel to the cleavage, and enclosing apatite, magnetite, augite, etc. 

 On the right a large plate of biotite embedded in a groundmass of little augite prisms, biotite, magnetite, and 

 melilite. 



Fig. 6. Melilite monchiquite, Long Geo, Holm, Orkney — magnified 200 diameters. Augite in prismatic 

 crystals and magnetite are embedded in clear melilite, which at the upper margin is decomposed, but on the 

 left, a little above the centre, shows the peg structure. In the centre and on the right the pegs are not 

 exactly in focus, but are indicated by the parallel striae which cross the mineral. 



VOL. XXXIX. PART IV. (NO. 33). 6 Z 



