20 ScientijiG Proceedings, Royal Dublin Society. 



which has brought down the great schist series against the Lough 

 Keel and Knockalla quartzites between Muboy Bay and Lough 

 Swilly. However there was, subsequently to this fault, an np- 

 thrusting from the south-east, which has pushed the Knockalla 

 quartzites on to the younger schists, thereby increasing the 

 complications. 



The complications in the country west of Lough Swilly are so 

 considerable and intricate, that it would be nearly impossible to 

 determine with certainty in that district by itself, the number and 

 the true relations of the different series of the Later Period Mocks. 

 Here some facts would suggest that the Lough Salt quartzites, 

 with their overlying schist series, were quite distinct from, and 

 older than, the Lough Keel quartzites and Milf ord schist series ; 

 also that the Killygarvan volcanic series was of considerable thick- 

 ness. In Inishowen, however, to the east of Lough Swilly, these 

 rocks lie in their regular order in four or five different sections ; 

 that which is best exposed being the coast section to the north- 

 west of Buncrana. In these sections we have a key to the compli- 

 cated tract to the westward in Kilmacrenan.^ 



Before and during the accumulation of the Later JRocks the sea 

 was of unequal depths. This is evident if the rocks be traversed 

 north and south, from Knockalla to the Swilly river, as different 

 groups of rocks are found resting directly on the Older Period 



^ My first visit to the Malin promontory was under adverse circumstances, which 

 led to a mistake on my part. At Lag, to the west of the promontory, there was 

 evidently the basal Great quartzite; from thence for a considerable distance to the 

 Coast Guard Station very little rocks are exposed, while further north to Malin Head 

 the rocks evidently belong to the Older Period. Griffith's boundary is a little north of 

 the Coast Guard Station ; and from the road, the rocks in the little headland are seen to 

 dip southward, while the rocks on the adjoining shore dip northward. At the time I 

 could hardly walk, so I took Griffith's boundary to be correct. Since then I have re- 

 visited the place, and have found that at the headland, a little north of the Coast Guard 

 Station, there is an anticlinal curve in the gnoissose quartzite, while along the axis of 

 this anticlinal there is a whinstone dyke, the anticlinal and the dyke giving the appear- 

 ance of an unconformabHity in the cliff, as viewed from the road. During my second 

 visit I examined the west coast carefully, and traced the gneissose quartzite Math its 

 accompanying granite intrusions southward to the Great quartzite at Lag. The latter 

 extend nearly continuously across the promontory to the eastern coast and dip southward 

 at a high angle, while the gneissose quartzite to the north of it is undulating, generally 

 at low angles, thus, although of no great thickness, occupying a considerable stretch of 

 country. 



