the Counties of Mayo and Sligo in Ireland. 161 



G. Mica Slate and Hornblende Slate. H. Granite. 



Mica slate and its subordinate strata constitute the prevailing' rocks in the 

 Ox Chain and Erris Mountains. At the sea shore below the western declivity 

 of the hill of Tarmon, which forms the southern point of Mullet, the slate is 

 penetrated by granite veins. An instructive example of this interference 

 appears in a small cove, about four yards wide, near a point called Toneadoon. 

 At a bifurcation of the vein, a small detached fragment of mica schist, is en- 

 tangled in the granite, the laminae being in a direction different from those 

 of the surrounding schist. The vein is seen at both sides of the inlet, and 

 it has rendered the schist looser in its texture, as well as somewhat friable; 

 the laminated structure also is partially obliterated, and the mica is replaced 

 by a mineral resembling hornblende. This change takes place gradually in 

 the neighbourhood of the vein, which is highly crystalline, the felspar being 

 in large rhomboidal crystals of a flesh red. The granite is laminated, the 

 mica is greenish white, and the quartz nearly translucent; and the layers are 

 parallel to the sides of the fissure, but coarser and larger in the middle. 



It is evident, the rock has been disturbed subsequently to the formation of 

 these veins, as a vertical slip has almost broken their continuity near the ex- 

 tremity next to Tarmon Hill. The mica slate there dips 70° to the south, and 

 appears to have been shifted about half the breadth of the vein. It contains 

 also veins of quartz, sometimes four feet wide. The junction of this rock with 

 the granite, can also be examined at an inlet north of the signal tower, the mica 

 slate dipping 15° to the east, and the edges of the beds abutting unconforraably 

 against the mass of granite. At Doonaniron, north of Broadhaven, the 

 quartz rock dips 60° to the south, and upon it the mica schist lies conform- 

 ably, but distinctly separated ; the quartz beds being unusually pure and 

 unmixed with extraneous matter, and the schist abounding remarkably in 

 mica. Mica slate prevails through the district south of Kilcommon, as far as 

 the banks of the Owenmore ; and continuing thence to the river Munneen, 

 the boundary probably ranges by the foot of Corslieve, Nefin, Knock-na-Ree, 

 and Shraheen Mountains, to the Ox Range, where, in the gap of Lough Talt, 

 beds of the schist approach in composition hornblende rock. But this is 

 merely a local variety. Above Lough Eask garnets abound, and pinite some- 

 times occurs ; and to the eastward asbestos is found. 



On the north coast, mica slate forms the eastern cliff at the entrance of 

 Portacloy, where the beds are most capriciously contorted by the influence 

 of the first dyke; and it extends to Porturlin, but is there succeeded by quartz 

 rock, which prevails to Benmore. At this point mica slate reappears, rifted 

 and disturbed by the intrusion of trap, which rises perpendicularly from the 

 sea two thirds of the height of the cliffs, and then turns off for a short distance 



VOL. V. SECOND SERIES. Y 



