466 DE. A. JOWETT ON THE [Oct. I9I3, 



About 15 yards from the cliff, the dyke is cut off abruptly against 

 the Upper Old Eed Sandstone by the main fault, which is con- 

 tinued along the shore. A smaller fault in the lava, parallel to 

 the main fault, and nearer to the cliff, throws the dyke 18 inches 

 horizontally westwards. This dyke is unquestionably of Lower 

 Old Eed Sandstone age. Some 50 or 60 yards west of it another 

 important fault occurs, which causes a small inlet and trends 

 north 57° ea3t — almost parallel to the main fault (north 62° east). 

 We see, then, that a number of faults having the same general 

 trend, and evidently the result of closely-related earth-movements 

 — probably all throwing down south-south-eastwards, and effecting 

 a slight horizontal westward movement of the rocks on their 

 southern sides — occur between the Bock of St. Skae and the bay 

 west of Boddin Point. 



Now, the general trend of the coastline, from Fishtown of 

 Usan to the Lunan-Bay sands, is closely parallel to the general 

 trend of this system of faults, and it seems very probable that the 

 similarity is more than a mere coincidence. The significance of 

 these facts will be more apparent when we come to consider the 

 system of faults parallel to the line of cliffs on the south side of 

 Lunan Bay. 



To return to the volcanic rocks west of Boddin Point. Nearly 

 all are very much weathered, some too much so for proper deter- 

 mination. The one against which the Upper Old Eed Sandstone 

 is thrown is a much-weathered amygdaloidal rock with felspar- 

 phenocrysts, and is probably an olivine-basalt. On the northern 

 side of the fault, in the small inlet 100 yards farther west, an 

 olivine-basalt without felspar-phenocrysts occurs, but this is 

 immediately overlain by a type containing obvious felspars. There 

 is a remarkable development of the sandstone-filled fissures and 

 clinker-beds among all the lavas in these cliffs. Usually, well nigh 

 half of the total thickness of each lava-sheet is composed of such 

 materia] ; but, sometimes, the lava is so completely penetrated by 

 veins of sandstone that very little unbroken compact volcanic rock 

 remains. The total thickness of a complete sheet in this neigh- 

 bourhood is only about 15 feet on the average. Farther west much 

 thicker sheets occur, especially near the outlet of Dunninald Den. 

 Here an important north-and-south fault throws down westwards 

 a thick sheet of enstatite-basalt, which appears to extend without 

 a break from high-water mark nearly to the top of the cliffs. 

 A rock very similar to this occurs on the eastern side of the 

 ' Den,' about a third of the way up the cliff, where it rests upon 

 olivine-basalt. 



I could find no trace of the ' dyke ' marked on the Geological 

 Survey 1-inch map, but a peculiar silicified breccia occupies the 

 position of what I take to be the fault mentioned above. 



The cliffs (Eickle Craig) from south of Dunninald Den to the 

 Lunan-Bay sands are high and rugged, exhibiting some fine ex- 

 amples of the effects of marine erosion on rocks of this character. 



