866 MR JOHN S. FLETT ON 



that these dykes belong to the bostonites, camptonites, monchiquites, and alnoites, and 

 that we have in Orkney a very interesting and remarkable series of this peculiar group 

 of rocks. 



I. The Geological Features of the Trap Dykes. 



Their Number and Distribution. — The number of dykes from which specimens have 

 been collected and examined during this investigation cannot be less than a hundred, 

 and this might be considerably increased were we to regard as separate dykes what are 

 really the branches and offshoots of one. Where two dykes are seen close together and 

 running in parallel courses, it is not always possible to say definitely whether they are 

 distinct, or would unite to form one, could they be traced for some distance. And there 

 can be no doubt whatever that this is merely a fraction of the total number in the 

 district. Far the most satisfactory exposures are those to be obtained by following a 

 bare and rocky shore which runs in a direction at right angles to their trend, such as, 

 for example, the west coast of the Mainland. Where the beach is low and covered 

 with gravel, a dyke may easily escape notice, or be quite concealed, even though the 

 occurrence of trap in loose blocks may indicate its presence. And, in the inland districts, 

 it can only be a very small minority which meets the eye of the geologist. They are to 

 be found in quarries, in burns, on the shores of inland lochs, and very occasionally 

 they can be traced in a roadside or on the bare slopes of a hill. But in the cultivated 

 fields, or the low moors covered by peat or boulder clay, any number of dykes might 

 be so effectively concealed as to yield no evidence of their existence. 



The rocks of which they consist are, as a rule, too soft and too readily decomposed 

 to enable them to form outstanding wall-like outcrops. Such, however, do'occasionally 

 occur, as at Netherton (near Stromness), Crowness (near Kirkwall), and in the north of 

 Birsay. They are usually denuded to the level of the surrounding flags, or lie even in 

 trenches formed by their rapid decay. It is very rare to find that any noticeable 

 feature of the scenery can be attributed to their presence. On the lofty cliffs of the 

 west coast they sometimes, by reason of their perfect jointing, give rise to a narrow 

 " geo " with parallel sides. On Rowe Head, near Skaill, the flagstone between two 

 parallel dykes has been tunnelled out by the sea, forming a natural arch, the Hole of 

 Rowe, the sides of which are lined in part by the trap dykes. Occasionally a dyke is 

 so resistant as to form the face of an exposed cliff, as, for example, at Skaill in Rousay. 

 In the inland streams their presence is commonly marked by small waterfalls, but at 

 Corrigal, in Harray, the burn flows in a deep, narrow trough, out of which a trap dyke 

 has been washed. The burn has evidently been turned out of its preglacial valley by 

 a dam of boulder clay, and has followed the outcrop of the trap dyke in cutting its 

 new course. 



Sufficient is known, however, to prove that their distribution is far from uniform, 

 and that from some districts they are entirely, or almost entirely, absent ; while in 

 others they are so numerous that several may occur in a distance of a few hundred 



