A. H. Green — Geology of Co. Donegal. 557 



inclined rocks, showing the same undoubted alternation of layers 

 of coarse and fine rock so clearly marked that no one could have 

 distinguished them from the laminas of an unaltered stratified rock. 



I am quite sure that this structure was not foliation ; some of the 

 layers may have been rudely foliated in themselves ; but along the 

 planes separating layer from layer there was no gathering of any one 

 particular mineral, such as mica. The structure, indeed, was exactly 

 that which is universally accepted in the case of a cleaved slate rock 

 as proof of its originally bedded nature, an arrangement of the rock 

 in parallel layers of various thicknesses and different mineral 

 composition, grain, and colour. 



One other point of resemblance between these bedded granites 

 and unaltered stratified rocks deserves notice. I have compared 

 them to sandstones. Now it is well known that in sandstones the 

 most persistent layers are those which are finest in grain, and that 

 the coarse beds are generally wedge-shaped, and thin away often 

 very suddenly. This was just the case with these granite laminae. 

 I found many cases of a coarsely grained bed wedging out sharply, 

 while the beds of finer grain were regular and persistent in their 

 thickness ; and I was constantly reminded among these granite hills 

 of the behaviour, in this respect, of the Carboniferous sandstones 

 of the North of England, from which I had just come. In both 

 cases coarseness of grain went along with irregularity of bedding, 

 and the contrary. 



To give more instances of detailed observations would be mere 

 repetition. The two cases I have selected were the most marked 

 that came under my notice ; but it was not on the strength of these 

 two alone that I formed my conclusions. The same thing was seen 

 wherever I entered the granitic area. 



One very obvious objection deserves notice. The layers of granitic 

 gneiss, which I have looked upon as interbedded with mica-schist, 

 and other undoubtedly stratified rocks, may be sheets of intrusive 

 igneous rock injected between the planes of bedding. The possi- 

 bility of this occurred to me, and I kept my eyes open for any 

 instance of the granitic gneiss cutting across the bedding, a thing 

 which was sure to occur somewhere, if it were really an intrusive 

 rock. I never saw anything of the kind ; and it is hard to believe 

 that, if these crystalline rocks have been injected from below, they 

 should everywhere have confined their lines of exit to the spaces 

 between two adjoining beds ; and if the main granitic mass be itself 

 intrusive, it would surely somewhere or other have thrust out dykes 

 and veins into the beds which it invaded. Moreover, we could not 

 in this way explain away the bedded structure of the main body of 

 the granite. A little further to the west a large mass of intrusive 

 trap does occur, and it is instructive to compai'e what is seen along 

 its margin with the border ground between the mica- schist and 

 granite. The boundary of the trap is a winding line running in 

 and out among stratified rocks, whose strike is all but invariable ; 

 big wedges spring from tlie main mass ; and innmnerable dykes, 

 many only a few feet wide, and probably occupying the openings 



