the Counties of Mayo and Sligo in Ireland. 159 



The derangements represented in the sketch might have resulted from repeated slips of the 

 strata in varying directions ; suppose (see diagram No. 2.) the aggregate of beds marked A 1, 2, 

 3, 4, to have slid together in the direction E E, the interruption at C would be the result; after 

 which a series of successive slips of the individual beds A 1, 2, 3, 4, in the altered direction F F, 

 parallel to the vein C C but oblique to D D, would displace the latter in each bed, as represented in 

 the plan ; while G G, in consequence of its position, so much inclined to the planes of those strata, 

 would be differently affected, and the breaks in its continuity might be scarcely perceptible*. 



No. 2. 



A great part of the coast from Broad Haven to Glen Lassera consists of 

 quartz rock, varying considerably in dip and direction. East of Broad Haven, 

 from Kilgaligan to Kid Island, the beds are nearly vertical, and of pure white, 

 granular quartz, in w^hich I saw no vestige of imbedded minerals. At Sheea- 

 nemore, between Porturlin and Balderig, is a remarkable slip, the western 

 portion of the cliff being depressed about 300 feet. At Glen Lassera this for- 

 mation disappears beneath the intermediate rocks, and recurs no more along 

 the coast to the eastward. 



F. Gneiss. 

 This rock is well characterized at the Pontoon Bridge between Lough 

 Conn and Lough CuUen ; also at many places in the peninsula of Erris, and in 

 the mountains at MuUi na Shee above Coolany ; but sometimes it graduates 

 into mica slate. At the Pontoon the mica is black, and the foliated structure 

 is strikingly displayed; while the occurrence of imbedded crystals of red fel- 

 spar, gives a porphyritic character to the rock. The beds dip from 10° to 45°, 

 and 60°, to the south-west, the prevalent direction throughout the Ox Chain, 



* The explanation offered in the text accords with that proposed by the late M. Schmidt, to 

 account for the phenomena of veins having been apparently shifted in opposite directions, with- 

 out any intermediate break (1 C. D 1, Exposed Section, and wood-cut No. 1.); and depends upon 

 the strike of the veins not being parallel (C C and D D of Plan). In the above case, at My- 

 creeny, the movements are supposed to have been in the direction of the arrows, or towards tiie 

 front of the cliff (No. 1. wood-cut) ; and the upper or shifted portions to have been removed. 



