THE TRAP DYKES OF THE ORKNEYS. 371 



parallel dykes of smaller^breadth, and deviates occasionally from its straight course by 

 passing for a short distance along cross joints. The microscope shows that they are all 

 very similar, and belong to a peculiar group of the camptonites, with abundant 

 felspar. 



Now, in the beach, below high-water mark, and just north of the cottage which 

 stands between the burns, three other dykes are to be seen, 3 feet, 1 foot 6 inches, 

 and 2 feet wide, running nearly straight in parallel courses, but in a direction almost 

 at right angles to that of the camptonite dyke described (N. 20° E.). They are 6 feet 

 and 30 feet apart, but they are to be regarded as really one dyke, and belong to a very 

 basic type, the biotite-monchiquites or alnoites, and contain no felspar. They are 

 exposed for 40 or 50 yards, and, as we trace them southwards towards the little cottage, 

 the east dyke thins, and finally dies out in a small vein ; the middle dyke twists a good 

 deal in its course, and approximates gradually to the west dyke, which is the most 

 constant of the three. The west and mid dykes are finally covered by the gravel of 

 the shore just behind the cottage. In the manner in which the east dyke dies out in 

 a thin vein, while the other dykes twist from side to side and send veinlets into the 

 chinks of the flags between them, which behind the house are filled with little 

 ramifying jets of trap, we have something so different from the usual straightforward 

 behaviour of the trap dykes as to prove that some peculiar obstruction lay in their 

 course. This must have been the camptonite dyke which runs beneath the cottage, 

 and which would necessarily bind the flagstones together and render irregular the series 

 of cracks into which the later dykes were injected. The actual junction is covered by 

 gravel and by the cottage, but there can be no doubt that we have here dykes of two 

 different series, running each with the characteristic trend of its group, and crossing 

 one another, and that the more basic were later in origin. 



From no other locality has evidence been obtained which would elucidate the 

 question of the succession in time of the different types. From the way in which the 

 monchiquites are interspersed among camptonite dykes, agreeing with them in trend 

 and in other characters, it seems, on the whole, most probable that they are of closely 

 allied origin and of very similar date. Like the alnoites, the bostonites have a dis- 

 tinctive course, which would point to their having had a separate period of injection, 

 but nothing is to hand to show whether earlier or later than the others. 



The Age of the Dykes. — It has been generally assumed that the trap dykes of the 

 Orkneys are outlying members of the great Tertiary series so well developed in many 

 parts of Scotland. That they are not basaltic does not disprove this, as, in the opinion 

 of Brogger (VI., p. 26) and others, the camptonites may be products of a basaltic 

 magma. On the other hand, there is no conclusive evidence to establish this opinion. 

 They cut the highest rocks of the Orcadian Old Red Sandstone in the district, the red 

 sandstones of the John o' Groats series, near Newark Bay in Deerness. They are not 

 yet proved to be later than the Upper Old Red Sandstone of Hoy; they are not 

 confined to the Orkneys, as, in a collection of rock sections from Shetland, kindly lent 



