Ixxiv PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



or the basic and the acidic crystalline rocks, founds his reasoning 

 on a comparison of the Tiltimate composition of the granites with 

 certain lavas of Monte Nuovo, with pumice and obsidian, whilst he 

 conyeniently ignores the differences of composition which tell against 

 him. The simplification of the history of the earth which M. Favre 

 claims as the merit of his hypothesis, will not, I fear, rescue his views 

 from the severe onslaught of critics who, whether they belong to 

 the Plutonic or to the Metamorphic school, will be slow to accept a 

 doctrine based on an unproved intermingling of rock-substances 

 essentially different in mineral constitution as well as ultimate 

 composition. 



M. Favre's statements on the petrological characters of the central 

 parts of the Mont Blanc chain recall the conclusions arrived at, after 

 a long series of analyses, by our talented associate the Eev. Dr. 

 Haughton, who first, as I believe, discovered oligoclase to be an in- 

 gredient of some of our British granites, and drew an interesting 

 parallel between those of Donegal, in Ireland, and the analogous 

 rocks of Canada, Sweden and I^orway, and Mont Blanc. "With 

 regard more particularly to Ireland, I am indebted to Dr. Haughton 

 for the following resume of his conclusions on this class of rocks. 



The granites of Ireland are divisible into tliree distinct groups :-— 



I. The granites of Leinster. 



II. The granites of Moui^ne and Carlingford. 



III. The granites of Donegal, Mayo, and Galway. 



I. Tlie Granites of Leinster. — These granites are, geologically, 

 newer than the Lower Silurian, and older than the Carboniferous 

 strata. Mineralogically they are identical with the granites of 

 Cornwall and Devonshire. They are composed of : — 



1. Quartz. 3. Margarodite. 



2. Orthoclase. 4. Lepidomelane. 



They are therefore, in composition, quaternary granites ; and their 

 paste probably contains minerals different from those found crystal- 

 lized in distinct masses. Like the Cornish and Devonshire granites, 

 they are occasionally traversed by mineral lodes, particularly lead- 

 lodes, which seem to have been formed in both countries at the same 

 geological epoch. 



II. The Granites of Mourne and CarJinc/ford. — These granites 

 are, geologically, newer than the Lower Silurian formation, and 

 also newer than the Carboniferous Limestone, which has altered 

 them into remarkable syenites on theii^ southern and western flanks ; 

 mineralogically, they are composed of : — 



1. Quartz. 4. 'White mica (margarodite ?). 



2. Orthoclase. 5. Black or green mica (lepidomelane ?). 



3. Albite. 



They are, therefore, quinary granites, and differ from aU granites 

 hitherto described by mineralogists in containing aJbite. These 

 granites, when they have intruded into the Carboniferous Limestone 

 on their southern and western borders, are converted, by a species 

 of endo-metamorphism, into syenites of different kinds — more espe- 

 cially, near Carlingford, into a syenite composed of augite and 



