ON A HILLSIDE IN DONEGAL 355 



movement of the material that fills the volcanic diamond-pipes, 

 reminds us that " eclogites " — i.e. crystalline rocks consisting 

 mainly of garnet and pyroxene or garnet and amphibole — are 

 among the commonest inclusions — or segregation-blocks, which- 

 ever we prefer — found in gneiss or granite. 1 In many cases 

 they have been traced to their parent-rocks, which prove often 

 to be limestones ; and their occurrence thus gives useful aid 

 to those who oppose the view of their origin by magmatic 

 differentiation within the granite mass. 



Among the best-known rocks of the eclogite class are the 

 so-called " trap-granulites," or "pyroxene-granulites," of Saxony. 2 

 These are fine-grained masses, sometimes schistose, closely 

 resembling the " amphibolites " and " pyroxenites " with garnet 

 that are common as inclusions in our Irish granitoid gneiss. 

 They occur in elongated lense-like forms of all sizes, sometimes 

 by hundreds, within the pale Saxon " granulite." The latter 

 rock, which has clearly metamorphosed the neighbouring schists, 

 is an intrusive mass allied to granite. At one time it was held 

 that the contact schists were very ancient, and that, by dynamic 

 metamorphism, an already cooled granite had been forced in 

 among them, the whole series becoming rolled out into a banded 

 and gneissic complex. Now, however, geologists have been 

 brought to ascribe the alteration of the schists to the igneous 

 intrusion of the granulite, 3 and to regard the " trap granulites " 

 as pieces picked off from the invaded rocks and partly digested 

 in the normal granulite. Instead of being fundamental and 

 Archaean, the gneissoid granulite becomes a granite with fluidal 

 and contact structures, which intruded into strata as high in the 

 series as the Carboniferous. 4 



W. A. Humphrey 5 has recently found that the " Urgneiss," 

 or so-called fundamental formation, in Styria has affected, by 

 contact metamorphism, conglomerates of Carboniferous age ; 

 and we thus meet another instance where a gneiss, once believed 

 to be of high antiquity, becomes placed in the group of gneisses 



1 This matter is discussed in Trans. R. Irish Acad. vol. xxxi. p. 460. See 

 also Doelter, Pedogenesis (1906), p. 177. 



2 See discussion in Lepsius, Geologie von Deutschland, 2ter Teil (1903), 

 pp. 146, 169. 



3 Ibid. p. 171. 



4 M. H. Credner has presented a remarkable summary of the modern view in 

 the Compte rendu du Congres geol. intertiat. Vienna (1904), p. 116. 



5 Jahrbuch d. k.k. geol. Reichsa?istalt, 1905, p. 363. 



