THE TRAP DYKES OF THE ORKNEYS. 869 



porary deviations. Instances of curvature are rare ; one was observed by Professor 

 Heddle on the west coast, south of Skaill (at Borwick). " From a W. by N. it suddenly 

 curves round to a W. S. W. , and as suddenly regains its original direction, exhibiting 

 thus a bold sigmoid feature" (V., p. 119). On Rowe Head, Skaill, there is a dyke well 

 exposed on the bare cliff top for about 200 yards. About 100 yards back from the 

 face of the cliff it forks into two branches — one of which maintains the original trend 

 of the dyke — a little S. of W. ; the other curves away and takes a nearly S.W. course. 

 These are the only instances of curvature which have come under my notice. 



It is very common, on the other hand, to find a dyke branching into two or 

 more which run in parallel courses a few feet apart, and may unite again in a short 

 space to form a single dyke. A fine example of this is to be seen below Wide wall, 

 South Ronaldshay. A large dyke, seven feet broad, is exposed in the beach at the 

 mouth of the mill stream. It runs nearly parallel to the bank which overlooks the shore. 

 A few yards further south it is found to be much diminished in breadth, while lateral 

 dykes accompany it on each side. For about 400 yards it can be followed, and its 

 general direction is well maintained, though it frequently passes laterally for a few feet, 

 then resumes its former course. Sometimes there are three, at other times four or even 

 more parallel dykes, but it is to be noted that the sum of their respective breadths is 

 always equal to that of the original dyke where first seen ; and under the microscope 

 they are all very similar in character. At the Oyce, Finstown, a dyke penetrates a bed 

 of contorted slickensided flagstones, in which the jointing is disturbed and irregular, 

 and in consequence it breaks up into a plexus of ramifying veins. The flag between 

 them is markedly indurated. On the west coast of Birsay, north of Skaill Bay, there 

 are several dykes, which, traced along their courses, are found to die out with a wedge- 

 shaped termination. In a parallel joint a few feet away a new dyke appears, beginning 

 in the same manner as the other ends, and increasing step by step as it diminishes. No 

 connecting vein can be seen. There is no doubt that in these cases the dyke has passed 

 laterally along a bedding plane, and indeed this is a common feature of the exposures 

 seen in vertical cliffs. 



In the course of my examination of the Orkney dykes, two points have impressed 

 me as particularly clear. One is that in every case they availed themselves of the 

 highly perfect joints by which the flags were intersected. It is very rare to find any 

 brecciation, and, except where the dykes branch, fragments of flagstone are never found 

 included in the intrusive rock. The other is that in no case has a dyke been seen to 

 pass laterally into a sheet or sill. This may be due to the ease with which the sedi- 

 mentary masses opened along the joint planes, or possibly also to the pressure exerted 

 by a great thickness of superjacent rock, which has been swept away by denudation 

 since the time of injection. 



The Trend of the Dykes. — Observations were made by means of a pocket compass of 

 the direction in which every dyke was running so far as exposed, or, in case of branching 

 or deviation, of the average course as near as could be judged ; and although it is not 



