ON A HILLSIDE IN DONEGAL 347 



Van Hise, who does not pretend to have covered the wide 

 field of foreign literature, even in his recent monumental work, 

 supports Bonney's views as to the origin of banding in schists 

 and gneisses of sedimentary origin. 1 On the other hand, 

 " shearing " has been called in by many authors to account 

 for the flow-structures in such gnarled rocks as that beneath 

 our feet in Donegal, regardless of the curves and wrinklings 

 that surely would have been obliterated or non-existent under 

 any form of crushing combined with movement. It is time, 

 therefore, to inquire further into the phenomena presented to 

 us in the field. 



In a large quarry opened in our gneissic series near Fintown, 

 in Co. Donegal, we are introduced more clearly to the mean- 

 ing of this particular " complex " of rocks. The foliation of 

 the whole is here vertical — that is to say, crystalline rocks 

 of different types run up side by side like walls, or like the 

 uptilted beds of a stratified series. Here is a layer of grey 

 gneiss rich in biotite, here a true mica-schist, here a band of 

 uncrushed quartzite. On the north side of the quarry we have 

 vertical bands of soft green shale, which prove to be decom- 

 posed basic igneous rocks ; these are kept from complete 

 denudation by bounding walls of the grey gneiss. So far, the 

 gneiss, which looks like a granite with a general flow-structure, 

 might be supposed to represent the most altered layers of a 

 great sedimentary series. 



But, higher up in the quarry, this grey gneiss cuts abruptly 

 across all the bedded and foliated structure, and appears in a 

 massive condition, with the jointing of an ordinary granite. 

 Blocks of quartzite remain in it, here and there associated, as 

 we noticed elsewhere, with clinging films of mica-schist ; but 

 the mica-schist seems mostly to have been destroyed. A huge 

 block of diorite, into which the gneiss sends off veins, is seen to 

 be entombed in the gneiss, which is here clearly an igneous 

 mass, intrusive in and absorbing its surroundings. 



We may be assured that the parallel structure in the lower 

 part of the quarry is truly original and sedimentary, if we 

 examine the vertical beds of quartzite and mica-schist — altered 

 argillaceous rock — in the country at no great distance. Where 

 granite invades these ancient strata, even their bedding some- 



1 "A Treatise on Metamorphism," Monograph xlvii. U.S. Geological Sii7"vey 

 (1904), P. 762. 



