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



angle from beneath the water, and slope up gently to the crest 

 of the next abrupt descent. It seems as if the ice had cascaded 

 down the steep faces, dug out a hole at the bottom of each, and 

 flowed out of the hole up the gentle further slope. I do not think 

 I ever quite realized Prof. Eamsay's theory till I saw these little 

 lakes, where the whole process is before you as if in a model. 

 Possibly the hollowing out of these lakes was the work of local 

 ice-flows subsequent to the general glaciation ; but this point I had 

 not time to go into. 



In the discussion which followed the reading of this paper, Mr. 

 David Forbes took a chief part, and I am glad of this opportunity of 

 replying to his objections. 



He states, 1st, that Mr. Scott and Prof. Haughton declare the 

 granites to be decidedly intrusive. We are not accustomed to rely much 

 on authority in scientific matters ; and, even if I had these eminent 

 names against me, it would not be absolutely fatal to my views ; 

 but, as a fact, nothing can be more strong and decided than the 

 expression of both Mr. Scott and the authors of the Eeport of their 

 conviction of the bedded character of the mass of these granites. 

 They do mention one or two cases where, to use their own words, 

 the granite is "apparently intrusive," so that the state of the case is 

 this : we have a great mass of granite, which almost everj^where 

 seems decidedly interbedded with rocks whose sedimentary origin is 

 not questioned ; in one or two cases we find granite apparently cut- 

 ting across the beds. What is to be our guide as to the origin of 

 this granite ? Are we to look to its general character on a large 

 scale, or are we to rely on a few local exceptions ? And these 

 exceptions are capable of the simplest explanation in two ways. 

 There may be two granites, one metamorphic, the other intrusive. 

 Or it may be that, while for the most part metamorphism has not 

 gone so far as to destroy the bedding, it has at some spots been 

 extreme enough completely to fuse the rock ; and the fused rock, 

 expanding by heat and urged on by the pressure of gases confined 

 below, has behaved intrusively. That a metamorphic rock may be 

 at places intrusive has been explained so often that I am almost 

 ashamed to have to repeat it. 



2nd. Mr. Forbes states that I start from the idea that if rocks lie 

 conformably on beds of undoubtedly sedimentary origin, it is a proof 

 that they are themselves sedimentary or stratified. I have not, that 

 I am aware of, stated this proposition either directly or indirectly ; 

 and I have heard of such things as contemporaneous or interbedded 

 traps or lavas. But even supposing that the rocks in dispute were 

 originally of this class, they were not poured out as granite, and 

 must have undergone metamorphism to reduce them to their present 

 condition ; and this is all that I have ventured to assert. I have 

 simply said that they are bedded, and contemporaneous with the 

 mica-schist and quartzite of the district; and whether they are con- 

 temporaneous sedimentary rocks, or sheets of contemporaneous lava, 

 matters little to the question we are discussing ; for in either case we 

 should require metamorphism to convert them into granitic gneiss. 



