SECTIONAL TRANSACTIONS— C. 331 



The majority of the pebbles appear to be of local origin, but many are 

 of igneous types which are unlike those in the Howgill Fells. Numerous 

 pebbles of red crystalline limestone containing crinoids and other fossils 

 cannot have been derived locally, and resemble very closely the Keisley 

 Limestone. 



The heavy minerals, though plentiful in the fine-grained deposits, com- 

 prise only a few types, but these are very interesting and include the 

 following : zircons (large, rounded generally, many purple), tourmaline 

 (generally rounded, brown, blue and a striking plum-coloured type), 

 rutile, apatite, anatase, hornblende, sphene, garnet and abundant ilm.enite. 



Dr.W. B. Wright. — On the occurrence of the pre-glacial shore-line in Achill 

 Island, Co. Mayo. 



The extension of the area over which the pre-glacial sea-level in Western 

 Europe can be shown to approximate to that of the present day is a matter 

 of considerable interest, and the writer therefore wishes to take the oppor- 

 tunity of recording the occurrence of an exposure of this shore-line in the 

 Island of Achill in the west of Ireland. The locality in which it can be seen 

 is known as the Cathedral Rocks, and lies about a mile and a half south of 

 the village of River. Here a wave-worn rock platform lies about 20 ft. 

 above the corresponding platform of the recent shore. On it are beach 

 gravel and rounded blocks locally overlain by blown-sand. The old cliff is 

 partially exposed and passes inland behind a bank of drift. Head or scree 

 from its degradation extends outward over the beach materials, and boulder 

 clay covers the whole, thus establishing its pre-glacial age. The level is 

 definitely higher than that of the corresponding beach of southern Ireland. 



Afternoon. 

 Excursion to Kilburn. 



Wednesday, September 7. 



Discussion on The origin of Igneous rocks (Prof. A. Holmes ; Prof. 

 H. H. Read ; Dr. A. Brammall ; Dr. G. W. Tyrrell ; Dr. A. K. 

 Wells ; and others) : — 



Prof. Arthur Holmes. 



From the fact that basaltic magma fails to produce rhyolites or dacites in 

 the inner Pacific, it is inferred that the magmas of these rocks are not normally 

 products of differentiation of basaltic magma. The association of acid and 

 basic rocks in kratogenic regions and the ' contaminated ' characters of 

 dioritic and granodioritic rocks in orogenic belts suggest that granite magma 

 arises independently of basaltic magma, except in so far as the latter may 

 serve as a carrier of heat. Peridotite magmas cannot be accounted for 

 otherwise than by refusion. The conditions that would favour the refusion 

 of crystal accumulations from basaltic magma would also suffice to produce 

 peridotite magma from the upper part of the Lower Layer of the crust. A 

 plausible explanation of the genesis of felspar-free alkali rocks is forth- 

 coming if a peridotitic parentage be assumed. Thus, in agreement with the 

 geophysical interpretation of crustal structure, the evidence of comparative 

 petrology leads to the conclusion that at least three types of parental magmas 



