238 C. CALLAWAY OlS^ TEE GEAiJflTIC AISD 



III. Peooe op Lateeal Theust. 



From the preceding descriptions, it will be seen that there are 

 two prevailing strikes in this region, one to the W.S.W., the other 

 to the S.W., and that these 'strikes are found in both groups. 



In Sg. 5, the strike of the Lough-Foyle series is W.S.W., between 

 Londonderry and Cashel HiLl ; but between Fahan and Ballynarry 

 Bay it is S.W., and this is the strike of the older series right up to 

 the granite, and in the granite itself. 



West of Malin the older schists strike sometimes W.S.W. and 

 sometimes S.W. ; but in the quartzose series further west the strike 

 is S.W. 



In the Kilmacrenan section (fig. 6 ), the trend is aLnost uni- 

 formly to the W.S.W. from the granite to the west coast. 



These observations are corroborated by an interesting section in 

 Croaghmore, a mountain W.J^.W. of Letterkenny. This elevation 

 trends east and west, in agreement with the strike of the strata, 

 which consist of strong mica-schists dipping steadily to the north, 

 with perhaps a few degrees of west. jS'ear the summit the schists 

 form a low escarpment facing to the south, and these scarp-faces 

 display intense contortions, with small folds reflexed to the east. 

 These two systems of folding must have been produced by earth- 

 thrusts acting in different directions and at different epochs. 



The facts indicated do not suffice for a complete induction, but 

 they are enough for my purpose. During one of the periods of 

 contortion, the thrust came from the S.E. or the jST.W., probably the 

 former. This pressure was capable of producing the foliation in 

 the granite and the south-west strike in the schists on its eastern 

 side ; for, as we have seen, it was violent enough to squeeze hard 

 grits into schist-like rocks, to crush strata into masses of angular 

 fragments with the divisional planes Ijmg in all directions, to pro- 

 duce cleavage in coarse grits, to contort rocks into closely appressed 

 folds, to throw over the folds, and sometimes to give rise to thrust- 

 planes. It was probably during this important corrugation of the 

 crust that the granite was squeezed up from below. At any rate, 

 the lateral thrust easily accounts for the gneissic structure, which 

 may have been in part acquired during the extravasation and com- 

 pleted after consolidation by a continuance of the pressure, or 

 entirely produced after consolidation. 



The production of regional foliation by pressure in the case of the 

 Donegal granite may have an important bearing upon the origin of 

 the crystalline schists. 



The Rev. E. Hill, F.Gr.S., has called attention* to a gneissic 

 structure in the granite of Guernsey and made a similar suggestion 

 in reference to some gneisses. 



SUMMAEY OF EeSCXTS. 



1. The granitic rock of northern Donegal, originally supposed to 

 be the result of the metamorphism of sediments, and recently re- 

 * Quart. Journ. Geol. Soc. vol. xl. p. 419. 



