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B. N. PEACH AND J. IIORNE ON THE 



peninsular tract. On the shores of St Magnus Bay at Brei Wick, the inter- 

 bedded and intrusive igneous rocks are not found in such close proximity ; the 

 junction between the two being concealed by a sandy beach. From the 

 admirable coast sections there is little difficulty in determining the geological 

 structure of the volcanic masses. They form a great syncline, the centre of 

 which is occupied by a coarse volcanic breccia or tuff and from underneath this 

 breccia there crops out a series of slaggy diabase-porphyrites, with occasional 

 beds of red ashy sandstones and flags. Such is the general arrangement of the 

 strata, though the succession is occasionally disturbed by faults of greater or 

 less magnitude. The order of succession is best displayed in the cliffs bounding 

 St Magnus Bay, and we shall therefore describe first of all the section between 

 Brei Wick and Stenness. On the west side of Brei Wick Bay, which is the 

 eastern limit of the interbedded volcanic rocks, the following section is visible. 



Fig. 1.- 



4 3 



-Section in Brei Wick Bay, Northmavine. 



At the east end of this section occurs a bed of coarse tuff (2), with bombs 

 of porphyrite, averaging 6 inches across, which is succeeded by finer tuff(l); 

 the strata forming a small synclinal fold. Towards the west they are under- 

 laid by red sandstones (3), which are pierced by a mass of pink quartz-felsite 

 (4), like the intrusive igneous rock to the east of Brei Wick. Fragments of the 

 sandstones are seen adhering to the felsite, which have been slightly indurated 

 by the intrusive mass. On the west side of the felsitic intrusion, the sand- 

 stones are repeated with a westerly dip, and they are succeeded by the beds of 

 coarse and fine tuff already described. These are overlaid in turn by coarse 

 ashy sandstones, in which masses of tuff are curiously intermingled with layers 

 of sand in the same bed. Grey sandy flags rest on these ashy sandstones, 

 which are abruptly truncated by a fault bringing in the porphyrites. For a 

 short distance the order of succession is disturbed by intrusive dykes; but near 

 Tang Wick Ness the lavas are seen dipping in a north-west direction. 



In the little bay beyond Tang Wick Ness a bed of dark purple diabase-por- 

 phyrite passes underneath coarse volcanic breccia, containing blocks of schist 

 and porphyrites ; the latter being most numerous. These included fragments 

 of schist were doubtless derived from the sides of the old vents, and though wo 

 cannot now point to the sites of the volcanic orifices, still the existence of these 

 blocks of schist in the tuffs clearly indicates that they must have pierced the 

 metamorphic rocks of the district. 



Not far to the west of this locality, pale slaggy porphyrites rest on dark 

 purple lavas; the latter being overlaid near Stenness by coarse volcanic breccia, 



