THE OLD RED SANDSTONE OF THE ORKNEYS. 417 



An outcrop of special interest occurs in a field 400 yards west of Smiddybanks. 

 Here, in an old gravel-pit, a face some ten feet high is exposed, now much broken down 

 by weathering. The rock is a coarse red sandy ash, with green spots. In it occur very 

 numerous sandstone and flagstone fragments, some as large as a man's head, — -the sand- 

 stones baked into quartzites ; the flags fused and slaggy on their surfaces, and with their 

 edges rounded. Materials such as these form a considerable proportion of the whole 

 mass. It seems unbedded, or rather the few traces of bedding planes showed a dip dis- 

 cordant with that of the surrounding sandstones. No similar bed crops out along the 

 shore, and the outcrop seems to be limited in area and rudely circular in outline, though, 

 as it occurs in the midst of cultivated land, its exact margins cannot be traced. It is 

 difficult to understand what this is, unless it be regarded as a small volcanic neck, the 

 mixed nature of its fragments being so different from that of the other ash beds, while 

 its position in the centre of the intrusive sheets and lavas and ashes already described 

 renders such a hypothesis, to say the least, highly probable. 



There can be no doubt that all these volcanic rocks owe their origin to the same 

 period of volcanic activity. Their situation, almost in the direct line between the Neck 

 of Huna and the lava of Haco's Ness, points to the existence of a north and south 

 fracture or line of weakness, which may be ascribed to the earth movements, which, 

 at the close of the deposition of the Rousay rocks of Orkney, introduced new types of 

 sediment and new forms of life. To the westwards, at any rate, no trace of similar 

 structures has been found. At two subsequent periods volcanic rocks rose to the surface 

 in this district : one series forms the lavas and ash beds of Hoy, described by Sir A. 

 Geikie. These, too, are of basaltic character, but they are separated from those we are 

 at present considering by a great conformity. The others form the tra/p dykes, which 

 traverse the flagstones mostly in an E.N.E. and W.S.W. direction. But these latter 

 are in no place connected with surface outflows, and differ so widely in structure and 

 composition from the rocks of Deerness and Shapinshay, as undoubtedly to have proceeded 

 from quite distinct sources. They are, in fact, chiefly developed in the West Mainland, 

 and are comparatively few in regions occupied by John o' Groats rocks. 



Watersound. ^r^7TZ/ttfc»^ 01d Head> 



Section 4. -K and S., through South Ronaldshay. 2 miles = 1 inch. 



The only remaining district of the yellow sandstones is the basin of the South Isles. 

 A complete examination of this area I was unable to overtake, but was compelled to 

 confine myself to the islands of South Ronaldshay and Burray, in which they occupy 

 the largest area of any of the South Isles, and very clear sections are to be obtained. 

 Here, also, the underlying yellow series is well developed, and passes down by means 

 of a series of flaggy passage beds into the grey flags, which at the south end of South 

 Ronaldshay contain the Rousay fossils. These passage beds are well seen on the south 



