THE TRAP DYKES OF THE ORKNEYS. 879 



moulded upon felspar. The structure is intersertal. In other parts of the section the 

 hornblende has more continuity, and is less penetrated by felspar, but it is everywhere 

 less idiomorphic than usual, and obviously later than felspar, around which it is very 

 commonly deposited. This is a reversal of the normal sequence in these dykes, and 

 indicates a transition to the diabases and proterobases. There is no second generation 

 of augite in the slides. It is noteworthy that the usual deposit of hornblende on the 

 surface of the phenocrysts does not appear, being replaced by plagioclase felspar. The 

 sequence of crystallisation must have been magnetite, ilmenite, apatite, olivine, biotite, 

 augite, with a little hornblende in parallel growth ; then plagioclase, and .finally 

 plagioclase and hornblende. 



The South Ronaldshay Dykes. — In South Ronaldshay occurs a group of dykes, in 

 many ways the most remarkable of any of the camptonite dykes of the Orkneys. 

 These include that already described as running in the shore at Widewall, and breaking 

 up by repeated branching into parallel dykes. The two others are in Hoxa, a few 

 hundred yards to the west of the landing-place. These dykes are all comparatively 

 broad (5 feet to 7 feet), and carry large crystals of hornblende, augite, and olivine ; 

 they are more basic than the general type of the camptonites ; some of them, in fact, 

 contain so little felspar and so much glassy base as to pass gradually into monchiquites. 

 I have retained them, however, in this class, as they form a natural group, closely 

 connected, and their general facies is that of camptonites with little felspar. 



They all contain olivine in phenocrysts, usually decomposed, but in sections taken 

 from the east dyke of Hoxa, .one foot from the edge, it occurs in crystals almost 

 quite fresh, and showing serpentine only along the borders and cracks. Augite 

 and hornblende are present in crystals up to one-half inch in length in the west 

 dyke on Hoxa ; in the east dyke and at Widewall they are smaller, but numerous. 

 The augite phenocrysts resemble those described in the Stromness and Kennibuster 

 dykes in most respects. Like them, they contain flakes of brown hornblende in parallel 

 growth, and have commonly a margin of a darker brown. Zonal structure is very 

 marked, and some of the crystals contain a nucleus of a bright green colour, which 

 seems to have suffered from corrosion, being only partly idiomorphic. This augite is 

 slightly dichroic in shades of yellow, green, and darker green ; it has a smaller 

 extinction angle than the brownish augite which surrounds it. In one section the 

 nucleus extinguished at 31°, the periphery at 44°. The central augite has also a greater 

 axial angle, and apparently a weaker dispersion. The hornblende is in large, short, 

 stout crystals, with evident traces of corrosion, and distinctly zonal. A darker margin 

 surrounds a paler centre, but there may be five or six zones of varying shades of brown. 

 As a rule, these are strictly parallel to one another. In pleochroism and other characters 

 they resemble the hornblende in the dyke on Scabra Head (PI. II. fig. 3). 



The majority of the phenocrysts are of a more complicated structure, and consist of 

 several zones of different composition succeeding one another in a definite order. In 

 the centre of the crystal we may have a rounded remnant of the green augite above 



