870 MR JOHN S. FLETT ON 



pretended that high accuracy was attained, sufficient care was taken to establish the 

 following conclusion. 



Of the camptonites, which form almost 90 per cent, of the whole, the average trend 

 is E. 25° N., and rarely do they differ from this by more than 10°. Their regularity in 

 this respect will be best seen from the tables appended. It has not proved possible to 

 break them up into groups, each with a characteristic direction. Their extreme range 

 is from E. 5° N. to E. 35° N., and two dykes within a hundred yards of one another 

 may differ by as much as 10°. If we neglect one or two instances in which the outcrop 

 is too limited to justify positive observations, or where in all probability it is a cross 

 vein and not the main dyke that is exposed to view, their uniformity in this respect is 

 so marked as to indicate with certainty that they are very closely allied in age and 

 origin. The single dyke of bostonite is distinguished by having the most easterly trend 

 of the whole series (east and west). On the other hand, the most basic dykes, the 

 biotite-monchiquites and alnoites, have a very distinct northerly trend — the dykes at 

 Quanterness N. 20° E., at the burn of Ireland N. 5° W., at Naversdale N. 20° W., the 

 average being almost a direct north and south. Finally, the monchiquites proper seem 

 to be connected with the camptonite series, with which they agree closely in direction, 

 having perhaps a slightly more northerly direction. 



Sequence of the Different Types. — The main road from Kirkwall to Stromness, four 

 miles from the former town, crosses the mouths of two little burns which are flowing 

 into the Bay of Firth. The streams are about 100 yards apart, and the road runs here 

 along the sea shore ; a little cottage stands just above the beach between the streams, 

 and nearer the east one. In the west burn, just beside the bridge which carries the 

 road across it, a large trap dyke is seen 6 feet 6 inches broad, which stands up boldly 

 and forms a little waterfall. It is running east 20° north, or almost parallel with the 

 Stromness road, and is a notable dyke, as the centre is coarsely porphyritic, with large 

 hornblende crystals up to an inch in length ; the edges are fine grained, It disappears 

 in the grassy bank of the stream, but if we pass eastward, cross the road, and examine 

 the sea beach below it, we find, a little further east, that two dykes appear in the beach 

 with the same course as the burn dyke, and respectively 4 feet and 2 feet 6 inches in 

 breadth. In character they exactly resemble the larger dyke in the burn, except that 

 they are not so coarsely porphyritic, and their united breadths are equal to that of the 

 burn dyke. It is clear that the burn dyke has swung northwards along a cross joint, 

 hidden by soil and road, and after crossing the road has resumed its previous direction, 

 but now as two dykes running a few feet apart. These two shore dykes can be 

 traced in the gravelly beach till they disappear beneath the cottage. Here they must 

 still further split up, for in the east burn, after passing the cottage, we see only a thin 

 dyke with the characteristic trend, but not over a foot in breadth. The other branches 

 are covered by gravel or boulder clay, and are not to be found. We have here, in 

 fact, a case exactly similar to that already described from Widewall, South Ronaldshay. 

 a single, rather large dyke, with the usual direction, which repeatedly branches into 



