232 



C. CALLAWAY ON THE GRANITIC AND 



call it shale. Some of it is black. Lower down the slope, in the 

 quarry west of Burnfoot, we have grit and black schist closely inter- 

 foliated, the folds being reflexed to the S.E. Some of the grit is 

 massive, and its contortion can sometimes only be determined by 

 observing the behaviour of the associated schists. 



Shore north of Pahan Station. — At Pahan we reach Lough Swilly, 

 and our section keeps to the eastern margin of this picturesque arm 

 of the sea up to Dunaif Head. Rock is well exposed on the shore, 

 grits and black schist being still the prevailing types. Some of the 

 latter is still more modern-looking than at Cashel Hill, and Prof. 

 Bonney agrees in thinking it is not far removed from phyllite. The 

 beds stand at very high angles to both 'NAY. and S.E., and are 

 sometimes vertical. The evidence of lateral thrust is very pro- 

 nounced. The strata are not only crumpled into numerous folds, 

 but in some places are crushed into masses of angular fragments 

 thrown together with their divisional planes lying at all angles. 

 This pressure is not evenly distributed. Por example, the grit in 

 one place is normal, differing only from a Palaeozoic grit in the 

 mineralization, entire or partial, of the matrix ; but at a little dis- 

 tance, where the crushing and contortion are more intense, the parti- 

 cles of grit are flattened, and strong folia of a silvery mineral (? talc) 

 come in between them, so that the rock approaches a schist in struc- 

 ture. It is not difficult to understand that if the squeezing process 

 were carried still further, the fragmental structure might be entirely 

 obliterated, and a true schist be formed. This suggestive case is 

 not unlike examples I have already described elsewhere*, where the 

 thrust of the Caledonian gneiss in Sutherland has converted quartzite 

 into quartz-schist. The overthrust in this locality is to the N.W., 

 and the same fact is to be observed at Buncrana, about two miles 

 further on. 



I did not see any limestone in the section between Derry and 

 Buncrana ; but the Culdaff limestone is on the strike of the rocks 

 east of Buncrana, and pieces of it, as well as of a similar variety at 

 Hlies, near Buncrana, have been kmdty furnished me by Mr. R. J. 

 Cruise, of the Irish Geological Survey. These limestones are dark 

 bluish-grey in colour, and are intermediate in degree of crystallization 

 between the Drumahoe limestone and the limestones of the crystal- 

 line schistose series. 



Between Buncrana Bay and Ballynarry Bay. — We commence this 

 part of our section at jSTed's Point. Again we come upon the black 

 schist. It is here associated with a hard quartzose grit and a 

 quartzose micaceous schist. The dip for some distance is to the 

 N.W. at 20°-30°. The same beds are repeated again and again in 

 a succession of reversed faults of the structure represented in fig. 4, 

 a bed of grit, underlain by a seam of the black schist or phyllite, 

 resting unconformably upon a coarse schistose rock, dipping at a 

 high angle in the same direction. Sometimes black schist is wanting, 

 and the grit is in contact with the coarse schist. In one place a 

 seam of the last-named rock is bent into a series of Y-shaped folds, 

 ^ Geol. Mag. Dec. 3, vol. i. p. 221. 



