Vol. 6g.] VOLCANIC ROCKS OF THE FORFARSHIRE COAST. 465 



waves a little below high-water mark, where it is about 30 feet 

 wide. The dyke is about 16 feet wide. The eastern side of the 

 ' Rock ' is horizontally slickensided over a large area, and hori- 

 zontal slickensides may also be seen in a cleft along the ridge 

 running in a direction north 6° west. In the cliff on the east, and 

 cutting across the ' Rock ' just behind the Chapel, another fault 

 occurs, which is almost exactly at right angles to this (north 

 82° east), apparently displacing it, and is slickensided almost 

 vertically, throwing the rocks on its southern side some little 

 distance westwards horizontally. About two-thirds of the way 

 across the little bay east of the ' Rock' more fault-breccia may be 

 seen at about high-water mark, and this fault is exactly parallel 

 to the trend of the ' Rock ' (north 6° west). 



The difference in character of the vein-stuff 1 parallel to these two 

 sets of faults also furnishes evidence that the east-and-west fault 

 is of later date than those running north and south. The dyke is 

 displaced by the east-and-west fault movement, and is brecciated 

 by both faults. Moreover, in a cliff-section in the gully north 

 of the Chapel of St. Skae, the dyke appears flanked on each side 

 by lava, the main fault-movement (indicated by breccia) having 

 occurred in the lava. It is evident, therefore, that the intrusion 

 took place prior to the initiation of the older north-and-south 

 faults. 



From the Rock of St. Skae to Boddin village, the lavas are 

 olivine-basalts, which contain plagioclase-phenocrysts and 

 monoclinic as well as rhombic pyroxenes. Excellent sections 

 of amygdaloidal lavas with sandstone-filled fissures may be seen 

 in the cliffs. On the shore in Boddin Harbour near high-water 

 mark, a fault which throws calcareous Upper Old Red Sandstone 

 against the volcanic rocks is well displayed in horizontal section. 

 The excellent vertical section of the same fault where it emerges 

 north-west of Boddin Point, and the Upper Old Red Sandstone 

 itself, have been well described by Dr. G. Hickling. 2 The surface 

 of the sandstone is slickensided, the striations inclining at 10° from 

 the vertical, and therefore the total effect of the fault would be, not 

 only to throw the sandstone down southwards, but also to move 

 it westwards to a slight extent. Parallel slip-planes occur in the 

 sandstone for some distance south-east of the main fault ; and, even 

 at the extremity of the headland 300 yards awa}-, there is a well- 

 marked fault which trends exactly in the same direction. 



Some 50 yards west of the corner of the bay where the large 

 fault emerges, a porphyrite-dyke is intruded in the lavas in the cliff 

 and on the shore. It trends north 8° west, thus being practically 

 parallel to the St. Skae dyke, which it closely resembles ; although 

 the lavas are not faulted alongside it as in the latter case. The 

 dyke varies in width from 3 to 6 feet, and is markedly finer 

 in grain for about 3 inches from each of its boundary-planes. 



1 See below, p. 480. 



2 ' The Old Red Sandstone of Forfarshire, Upper & Lower ' G-eol. Mag. 

 dec. 5, vol. v (1908) p. 404. 



