426 Correspondence — Bev. T. G. Bonney. 



others are more or less coarsely crystalline rocks, in which pyroxenic 

 minerals usually predominate. These rocks are protruded in larger or 

 smaller masses, and allied to them are dykes of a maculated basic rock, 

 one of the hybrid rocks of Durocher. These dyke rocks are very 

 undecided in composition, and in places may be classed as dolerite, 

 while in others they must be called either Felstone or Trachyte. 

 These maculated rocks seem to graduate into Dolerite and Augite, 

 similar to and probably of the same age as the Tertiary dolerites of the 

 Co. Antrim. A typical elvanoid rock belonging to one of these groups 

 (fourth or fifth) occurs at Goragh Wood (where it is extensively 

 worked), coming up as a mass through the older Newry granite. 

 This rock would answer the description for Nevadite, and possibly 

 may be one of the granitic rocks belonging to the trachytes of Antrim. 

 The rocks in the country about Carlingford Lough at present are 

 only partially known ; this, however, ought not to be for long, as they 

 have been carefully examined by Mr. Traill. 



In conclusion, I may mention that in the Mourne district to the 

 north of the Lough, Mr. Traill found some of the dykes similar to and 

 probably of the same age as the maculated dykes of the Carlingford 

 district, that at their margins suddenly changed into a vitrioid rock, 

 locally called Bottleite, that when examined by our colleague, 

 F. Kutley, F.G-.S., was pronounced to be Trachalite. This trachalite 

 in places assumes a fibrous structure, apparently somewhat similar 

 to that described in the obsidian of Ponza by Judd, and from fibrous 

 it seems in places to pass into a minute columnar structure, the rock 

 at the same time changing into anamesite or basalt. Judd seems 

 to be of opinion that this fibrous structure is due to extreme pressure ; 

 with this I cannot agree, as it may occur in places where the dykes 

 evidently occur filling shrinkage fissures. Many, indeed most, fibrous 

 varieties of minerals and rocks, seem to be due to crystalline 

 structure, the substance being deposited from solution ; this, however, 

 is not always the case, as in some instances the process seems to have 

 been somewhat similar to drawing out heated glass into hairs. 

 Such, however, could scarcely be due to pressure, and in many places 

 where observed it looks as if the foundations of the dyke had given 

 way, and that films between the consolidated portion of the dykes, or 

 one of its walls, had been drawn out while the dyke was sinking. 



"Wexfoed. G. H. Kinahan. 



GLACIAL EEOSION. 



Sir, — There are some points in Mr. Goodchild's interesting com- 

 munications on Glacial Erosion (Geol. Mag. pp. 323, 356), concerning 

 which I should like to make a few remarks. As I have not the 

 advantage of much knowledge of the principal district which he 

 describes, I cannot attempt to discuss them in detail, but as most 

 points in his description appear to me to be common to all similar 

 districts that I have seen, I venture to offer two or three general 

 criticisms. 



He objects (p. 328) to the theory which attributes the formation 

 of rock ledges mainly to fluviatile action, because of .(1) their height 



