262 Dr. C. Callaway — On Comparative Lithology. 



I regard Anglesey as the stepping-stone between the typical 

 Pebidians of South Britain and the hypocrystalline rocks of Leinster. 

 A sketch of a few Leinster sections will here be given. 



The Hill of Howth. — The rocks of this well-known promontory, 

 projecting into the Irish Sea north of Dublin Bay, have been referred 

 by Irish geologists to the Cambrian epoch. The lithology of the 

 strata, however, does not lend any support to this view. Taking the 

 section from north to south, we have the following succession : — 



1. Light-grey altered shales. 



2. Similar shales, interstratified with quartzite. 



3. Ibid, with pale-green schistose slate and purplish-grey shales. 



4. Pale-green breccia, fragments of green schistose slate like the 

 first variety in 5, matrix gritty, with schistose lustre. 



5. Pale-green and yellowish altered shales, described by Prof. 

 Bonney ^ as " rather talcose or serpentinous " ; interbedded with 

 quartzite and overlain by a great mass of the same. 



6. Quartzose grit interstratified with and overlain by pale-green 

 shales. 



The resemblance of these strata to the hypometamorphic (Pebidian) 

 series of Anglesey is A^ery marked. Quartzites are not known in 

 the Pebidian of South Wales ; but they occur in that group in 

 Anglesey. In Howth Hill they are still more conspicuous. On the 

 other hand, there is little likeness between the Howth rocks and the 

 purple and green aluminous slates and unaltered grits of Bray Head 

 and the Devil's Glen, which are usually placed in the Cambrian. 



CarricTc Mountain (Co. Wicklow). — This ridge is made up of 

 quartzite interbedded with yellowish, greenish, and purple mudstone 

 or shale with a north-westerly dip. These rocks are not unlike some 

 of the less altered varieties of the Pebidian, but I hesitate to speak 

 positively. 



Aughrim (Co. Wicklow).— I made the following notes of this 

 locality : — 



1. One mile N.N.E. of the town. Grey schistose shale ; dip 

 S.S.E. at 65°. 



2. One-eighth mile north of Macreddin Bridge and ridge east of 

 the same. Altered slaty beds; black subcrystalline schist; same dip. 



3. Quarter-mile S.S.E. of Three Wells. Similar altered slates ; 

 same dip. 



The rocks of this district have a marked Pebidian facies. 



Wexford District. — Two bands of newer Archaean and Lower 

 Palaeozoic rock, striking W.S.W., are separated by a strip of Car- 

 boniferous Limestone. 



The Northern Zone is largely composed of quartzite and other 

 altered rocks. Going in a north-westerly direction from Wexford 

 town, we pass over the following section : — 



1. Quartzite interbedded with black schists. 



2. Green shale or slate, much mineralized. 



3. At Carrick Bride, we have altered green slates. These appear 



^ In MS. notes kindly fui-nished by my desire. , 



