146 



Proceedings of the Royal Irish Academy. 



the modification which the original structure of the ground has under- 

 gone, through overwhelming earth-stresses operating laterally from 

 the east and south-east of the area under review. 



Fig. 9. — Section across Lougli Feeagh (W'll) showing quartzite (5), pebbly beds 

 (i), limestone {Is), and mica-schist (m) — natural order of succession 

 inverted. 



The structures described as manifest in I^orth Mayo are but 

 reproduced in jN'orth Galway. The quartzite of Maamturk Eange 

 aud the Twelve Pins is the most striking feature of the area, and in 

 general it dips northward, and under the mica-schist, cleaved grits, and 

 limestone of the comparatively low ground skirting the range on the 

 north. Prom our knowledge of the succession in Mayo, as well as 

 judging from overfoldings which are to be observed in the valley 

 slopes of the quartzite range, I have no hesitation in maintaining that 

 the apparent order of the strata on the north side of the Twelve Pins 

 is the converse of the original order of succession. This conclusion is 

 confirmed by finding that the mica-schist and limestone series which 

 underlies the quartzite, east and west of Letterfrack, is continuous 

 with the series around Kylemore, to the north of and overlying the 

 Twelve Pins quartzite. 



The boulder-deposit is strikingly developed at Cleggan Hill, 7 

 miles west of Letterfrack, where it takes up a zig-zag course in the 

 greenish-gray mica-schist beneath the quartzite which here forms the 

 hill (481). The pebbles range up to 12 inches by 3 inches in section, 

 and consist of quartz, quartzite, and a f elspatliic rock. In attempting 

 to trace this deposit to the eastward it was found to be represented in 

 a peculiar much-crushed band, half a mile north of the western end of 

 Eallynakill Lough, where the pebbles are drawn out into long lath- 

 shaped lenticles, embedded in a greenish-gray mica-schist. The con- 

 dition of the deposit here may suggest a reason for its non-appearance 

 further east for several miles ; it may have been crushed out of 

 recognition, as previously mentioned. 



Below the boulder-deposit at Cleggan, limestone beds occupy a 



