THE OLD RED SANDSTONE OF THE ORKNEYS. 415 



assigned to the upper part of the yellow beds. From this point westward they strike 

 along the shore, which they form up to the Bay of Newark, where they are covered by 

 blown sand. On the west side of the bay, beds occur striking N.N.E., and evidently let 

 down by means of a dislocation covered by the superficial accumulations in the centre 

 of the bay. The principal mass of volcanic rock at the Point of Ayre forms a narrow 

 area which runs E.S.E. out to sea, and is in breadth about 40 yards. Its base is not 

 seen, and its lower member is a thick bed of dark green volcanic ash, with large spherical 

 bombs up to 2 feet in diameter, vesicular, especially in the centre, and much decomposed. 

 A few bits of baked flag occur in the ash, and it weathers in a markedly spheroidal 

 manner, resembling, in fact, very closely many of the basaltic ash beds around the shores of 

 the Firth of Forth, as at Kinghorn and Elie. In general it shows no trace of bedding, but 

 here and there a few thin irregular lenticles of sand are to be seen, which prove that though 

 rapidly accumulated, it is not the product of a single outburst. A curious feature is 

 the existence in it of flagstone veins. These are very tortuous and irregular, an inch or 

 two in thickness, and filled with a normal, somewhat calcareous flagstone, in which little 

 or no trace of any metamorphism is to be found. They are vertical, and show no sign 

 of bedding or contortion, and are to be regarded as due to the formation of cracks in the 

 thick accumulation of volcanic ash, into which the ordinary sediment of the sea-bottom 

 was washed. At first glance, this bed of agglomerate suggests at once that it is a 

 volcanic neck, and the elongated form of its outcrop would support this explanation. 

 But its junction with the flags to the south is a small fault, and these show none of that 

 alteration which is to be expected in the walls of a volcanic neck. And, moreover, the 

 bed itself is seen in the low cliff to be overlaid by a thin lava, and that again by well- 

 bedded flags. Still, it is in every way probable that an accumulation of this sort was 

 formed in the immediate proximity of a volcanic orifice. The overlying lava is some 

 three feet in greatest thickness, vesicular at its upper surface, the vesicles being large, 

 not markedly elongated, and filled with calcite and other secondary minerals. It is 

 greatly decomposed, but shows little of the spheroidal weathering of the agglomerate, 

 being rather divided by well-marked joints into polygonal vertical columns. Under the 

 microscope it turns out to be an olivine basalt, so greatly decomposed that few of the 

 original minerals remain. At the western corner of the outcrop this lava is seen to be, 

 in turn, overlaid by ordinary flags, which are in nowise altered by the heat of the under- 

 lying rock, and contain little or no fragmental volcanic matter. These rocks are bounded 

 to the south, and probably also to the north, by small faults. A few yards to the west 

 of them, what seems to be a quite distinct outflow is exposed in the shore. This is the 

 edge of a small lava flow, three feet in thickness, and thinning out in a few yards to the 

 south, while the flags close over it. It is dark in colour, with large steam cavities in its 

 upper surface, and bears a striking resemblance to the volcanic rock at Haco's Ness, 

 Shapinshay. The sea has removed the overlying rocks, except at the thin edge, where a 

 layer of dark green ashes mixed with sand is seen to immediately overlie the lava, 

 succeeded in turn by a normal unbaked ordinary flag. The lava rests upon a similar 



VOL. XXXIX. PART II. (NO. 13). 3 R 



