Correspondence, 87 



atmospheric agencies. Various Traps weather freely, also some kinds 

 of Limestone ; Granites generally waste, more especially some of the 

 Granite Porphyries and Vein Quartz, which, when exposed, usually 

 split up and crumble away. 



If the atmospheric agencies (that is Chemical action, the Sun's rays, 

 Wind, ordinary Frost, Bain and Bivers) have formed the feattires 

 of this country, should not these kinds of rocks be those on which 

 they acted most ? I should answer " Certainly ; " but this is not the 

 case. The plain of Limerick is diversified by Carrigs, Bidges, and 

 Boons, most of which are composed of the Traps and Ashes that 

 are associated with the Limestones of that country. These Traps and 

 Ashes weather much more freely than the Limestones ; why, there- 

 fore are the hills formed of Trappean rocks ? Should not the Lime- 

 stones that are less affected by the atmospheric agencies be found in 

 the hills and the Trappean rocks in the low ground ? 



Most of the rocks in the hills that occupy the north-west part of 

 the County of Galway are dressed and scratched by Glacial action, 

 therefore the effects of weathering since that period can be well 

 observed. The Schist Quartzite and some of the Gneiss retain the 

 scratches well, but the scratches are rare on the other kinds of rock. 

 From this we see that the Limestones, Vein Quartz, Altered Traps 

 {Hornblende rocTc, Epidote, Granite, etc.), and Granites, weather 

 most, and therefore ought principally to have been eaten away, when 

 the features of the country were being formed by atmospheric 

 influence. This, unfortunately is not the case, as in most of the 

 summits of the hills Granite or Altered Traps are found. Besides, 

 associated with the Quartzite is Vein Quartz, full of minute joints, 

 which causes it to break up with each winter's frost, while the 

 Quartzite remains intact ; and yet when the features of the country 

 were formed, the former must have resisted better than the latter, as 

 the Vein Quartz now stands out in marked reefs and projecting 

 courses. 



The Limestones of the N.W. of the County of Galway, when 

 exposed, are deeply scored by the atmospheric agencies ; and as they 

 occur frequently in the large valley that extends from Oughterard to 

 Clifden it might be said — " This feature is due to the wasting away 

 of the Lime rock." However, I believe their occurrence here is 

 purely accidental, as the Limestones are not found in the whole 

 length of the valley, and in those parts of it where they do occur, 

 they are often in hummocks above the associated Gneiss and Schist. 

 A mass of Limestones strike across the hills N.W. of Oughterard, and 

 if the existence of the valley just mentioned is due to the waste of 

 the Lime-rocks, then Limestone ought also to have wasted away and 

 formed another valley. Moreover, on the north slope of the hills 

 that lie south of the Maum branch of Lough Corrib, there is an 

 isolated boss of this Limestone, forming a well marked Carrig on the 

 hill side. Ought not this Lime-rock to have disappeared before the 

 surrounding Gneiss and Schist ? 



These facts which I have mentioned, and that seem to me to be 

 against atmospheric agencies, would be in favour of marine denuda- 



