872 MR JOHN S. FLETT ON 



me by Mr Horne, of H.M. Geological Survey of Scotland, there is one of a much 

 decomposed camptonite dyke from Brenista Bay. Their great freshness, in many 

 cases, would point to a comparatively recent origin, for the igneous rocks of Old Red 

 age are in every case much more decomposed (VII., p. 412). But, till we know more 

 of the distribution of these rocks in the north-east of Scotland, it will not be possible to 

 arrive at any definite conclusion as to the geological period in w T hich they were formed. 



That they are later than certain of the faults by which the flagstones are intersected 

 has been shown by observations in two different localities. In the cliffs on the east 

 coast of Holm there is a narrow inlet, the " Long Geo," where the sea has eaten its 

 way inwards along the line of a fault. The " geo " runs east and west, and crossing it 

 obliquely, there is a small trap dyke, which is not brecciated or broken by the fault, 

 but, on the other hand, can be seen to twist out of its true course, and to split into 

 two branches, where it cuts the crushed fault rock. On the west shore of the Wart 

 Holm of Copinshay there is a beautiful monchiquite dyke, one side of which has been 

 exposed by the sea, and forms the surface of the cliff for a short distance. It differs 

 from all the other dykes in being not vertical, but inclined at an angle of 60° (to the 

 vertical plane). Along the shore there runs a fault, as is shown by a crushed and 

 slickensided belt of flagstones, and this has proved a line of least resistance, which has 

 been taken advantage of by the dyke. Although all the flags are much disturbed and 

 twisted, the dyke shows no deformation whatever, and is undoubtedly of later origin 

 than the fault. 



II. Petrographical Characters. 

 1. The Bostonites. 



The only dyke of bostonite known to occur within the area was found on Onston 

 Ness, a small promontory on the Loch of Stennis, just east of the Bridge of Waithe. 

 Its width is a little over two feet, and its colour is greenish-grey, and much paler 

 than is usual in the camptonites in the centre, but mixed with darker streaks ; while 

 at the edges, for about two inches, the rock is finer grained and darker in colour. In 

 the hand specimen it shows neither porphyritic nor fluxion structure, and contains 

 occasional vesicles which are filled with calcite. 



Sections were made of the edges and of both the lighter and darker varieties of the 

 -centre. In the centre the rock is porphyritic, having large phenocrysts of anorthocJase 

 felspar simply twinned on the Carlsbad plan, and with a hone3 7 combed appearance 

 indicating the former existence of glass enclosures (PI. I. fig. 2). These lie in a ground- 

 mass, rather coarse grained, of felspars mostly simply twinned, and almost isodiametric, 

 but without crystalline form. Among them are scattered long irregular sections o( 

 plagioclase crystals. This groundmass is holocrystalline, and fluxion structure is not 

 apparent. Calcite and limonite in irregular patches are scattered about the section. 

 There is no trace of quartz or of undecomposed ferro-magnesian minerals. But pule 



