^6 Proceedings of the Royal Ph^jsical Society. 



pale shales. The quartzites are traversed by joints in all 

 directions, which are abundantly coated with peroxide of 

 iron, and in many places they have a marked 'Schistose 

 character. We were fortunate enough in discovering an 

 abundant series of plant remains in these altered rocks, some 

 of which are tolerably well preserved. Mr C. W. Peach has 

 referred the plants to Psilophyton and Lepidodendron nothum, 

 and regards them as identical with the plants occurring in 

 the Old Ked Sandstone rocks of Caithness and Orkney. It 

 follows, therefore, that the rocks in which they are embedded, 

 altered though they be, must be relegated to this formation. 



Contemporaneous Igneous Rochs of Old Red Sandstone Age. 

 — In the western district of North Mavine, between Stennis 

 and Ockren Head, there is an important development of lavas 

 and ashes, associated at certain localities with ashy sandstones 

 and red flags, which belong to this period. These porphyrites 

 and tuffs resemble in every respect the volcanic rocks of the 

 same age in the Ochils. Excellent sections of these rocks 

 are exposed in the coast-line from Braewick to Stennis, and 

 thence to Ockren Head, where they have been tunnelled in a 

 wonderful manner by the action of the sea. The structure of 

 the area is comparatively simple, as the beds lie in a 

 synclinal fold, the dip near Braewick being to the north of 

 west, while along the western shore the porphyrites and tuffs 

 dip to the south of east. On the west bank of Eoeness Voe, 

 about a mile from the mouth of the sea loch, the porphyrites 

 are thrown against the intrusive quartz-felsite by a fault, and 

 in Braewick Bay it is highly probable that the same relation 

 exists between the two, though the evidence is obscured by 

 the sandy beach. 



In the Holm of Melby a bed of slaggy porphyrite occurs, 

 dipping to the west; and again in Papa Stour Professor 

 Geikie* has described a similar series of volcanic rocks, 

 exposed here and there along the base of the cliffs underneath 

 the sheet of pink porphyry. These are likewise associated 

 with beds of sandstone and conglomerates, and are doubtless 

 on the same horizon as the volcanic rocks of North Mavine. 



* See "The Old Red Sandstone of Western Europe," vol. xxxviii. Edin. 

 Roy. Soc. Trans., p. 345. 



