ON A HILLSIDE IN DONEGAL 34s 



but the view of the older geologists very generally was that 

 the streakiness represented the remains of stratification, and 

 that the gneiss arose from the c^stallisation of the materials 

 of normal sedimentary rocks. Consider our limestone bands, 

 our quartzites, our darker patches of micaceous or hornblendic 

 matter, all retaining a similar trend, or curving only with the 

 curvings of the structure of the gneiss. Do not these represent 

 residual masses that have escaped complete metamorphism ? 

 Do they not tell us of the obliteration of similar features in 

 the main body of the gneiss? It has thus been suggested 1 that 

 the blocks and balls of hornblendic rock embedded in a streaky 

 gneiss record for us the presence of an old conglomerate which 

 has become cr}-stalline through sheer antiquity. Moreover, 

 banded gneiss occurs again and again upon the margins of a 

 normal granite mass. Is it not reasonable to suppose that the 

 granite itself was once sedimentary, and represents to-day 

 the extreme phase of alteration ? 



A third view to account for the streakiness of the gneiss has 

 been generally held in recent years. Workers in regions where 

 mountains have been reared by lateral pressures above the 

 general level of the crust became impressed with the enormous 

 forces that operated upon the central cores. The granite that 

 is usually so prominent in these cores assumes a gneissose 

 aspect, and is succeeded by fine-grained schists, as one passes 

 from the centre towards the flanks of the mountain-chain. The 

 intense crumpling, over-folding, and over-thrusting that can be 

 traced among the stratified rocks of the foot-hills, led to the 

 belief that the structures of the crystalline core could be ex- 

 plained by similar deformation. The microscope gave valuable 

 aid in these inquiries, and proved that crushing and warping 

 have frequently affected the crystalline constituents of schists 

 and gneisses over very considerable areas. The parallel arrange- 

 ment of the crystalline materials, the " foliation," in fact, was 

 seen in many cases to be due to the action of pressure, and to be 

 absolutely independent of any previous structure in the rock. 



For instance, Judd writes in The Student's Lycll (1896,, 

 p. 549) : " There can be little doubt that, in the great majority 

 of cases, the schistose structure is an entirely superinduced 

 one, and that foliation, like cleavage, must be referred to the 



1 Hardman, " On the Metamorphic Rocks of Counties Sligo and Leitrim, and 

 the enclosed Minerals," Set. Proc. R. Dublin Soc, vol. iii. (1882), p. 358. 



