868 MR JOHN S. FLETT ON 



This rule, however, is not without exceptions. Near the Bridge of Waithe two 

 dykes occur — one a bostonite, the most acid of the whole series ; the other a biotite- 

 monchiquite or alnoite, one of the most basic. At Rennibuster, on the road from 

 Kirkwall to Stromness, camptonite dykes, rich in felspar, are associated with monchi- 

 quites, from which felspar is absent. In these cases, however, the trend is different, 

 and the association is merely fortuitous. 



Breadth, Jointing, Contact Metamorphism, etc. — The breadth of the dykes 

 may be stated to be on an average 3 feet to 3 feet 6 inches. Dykes under 

 a foot are rare, except as lateral veins proceeding from a larger mass. On the 

 other hand, one of 7 feet may be considered large. The dyke of Borwick, near 

 Yeskenaby, is 9 feet ; at Avalshay, Rousay, there is one of 10 feet, and the 

 broadest of all is one near the Spoord, on the west coast of Birsay, which 

 measures 12 feet from side to side. As usual in dykes, a series of transverse joints 

 divides the mass into horizontal angular prisms, and the jointing is commonly finer 

 and closer at the sides than in the centre. This is very well seen where the side of 

 the dyke is exposed on the face of a sea cliff. In the thicker dykes the jointing is 

 apt to be rude. Very often they weather in a spheroidal manner, exactly resembling 

 the diabases. This, as a whole, is most characteristic of the rather basic camptonites, 

 but it is far from universal. The general presence of fine-grained, chilled edges proves 

 that the flagstones at the time of injection were comparatively cold. Some of the 

 dykes, as will be later described, are remarkably coarsely porphyritic ; but this is con- 

 fined to the centre, the edges being fine grained, and, under the microscope, showing 

 traces of residual glassy material, even though they never have a vitreous appearance 

 to the naked eye. As remarked by Peach and Horne (III., p. 14), the dykes are often 

 vesicular, and the vesicles, which are filled with white calcite, tend to be central in 

 position. The flags adjacent to the dyke show usually a contact alteration, which 

 rarely extends to more than a few inches, and amounts only to induration with the 

 development of a closer texture and a splintery fracture. They frequently break 

 down under the hammer into small sharp-edged cubical fragments. Jameson (antea) 

 noted that where the flags contain much organic matter, bitumen may appear in the 

 vesicles. 



Regularity of the Trend, Branching, etc. — In most cases the exposures are very 

 limited, and it is always to be observed that a dyke follows one of the two series of 

 usually very perfect joints by which the flags are cut. These must have been ante- 

 cedent to the dykes, and furnished lines of weakness along which the flagstones opened 

 out. Wherever a dyke can be followed for more than a few yards, its course is found 

 to be not absolutely straight. It passes frequently from one joint to another by means 

 of a cross joint, then resumes its previous direction. This feature is so common that 

 instances need hardly be cited, but it is well seen in a dyke at the Brough of Birsay, 

 which runs parallel to the shore line, and can be traced in the rocky beach for about 

 400 yards, during which space it maintains its general direction with frequent tem- 



