Correspondence. 523 



Professor Ansted entered into some wide generalizations favour- 

 ing the metamorpliic origin of granite. I happen to reside in a dis- 

 trict where the intrusive character of that rock is particularly well 

 shewn, and I could hardly allow his views to pass unquestioned. 

 There is in this country the largest exposure, and perhaps the 

 greatest variety, of granite in the British Islands, and I trust that a 

 short account of my observations, which have been made with some 

 care, may not be without interest. 



There are four large tracts of granite in Ireland — (1) The Leinster 

 district, ranging from Dublin, through Wicklow, into Wexford ; 

 (2) the Mourne Mountain district, in the Co. Down ; (3) the 

 Donegal district ; and (4) the Connaught district. Granite also 

 occurs in smaller masses in other parts of this country. 



The Leinster granite (1) is unquestionably intrusive ; it pene- 

 trates into Lower Silurian Slates, which are everywhere altered into 

 mica-schist as they approach it, and are pierced by numerous granitic 

 dykes. The Mourne granite (2) has a similar character, though the 

 metamorphism of the surrounding rocks is not so extensive as in 

 Leinster : it is supposed to be a newer rock than that of Leinster, 

 being believed to be post-Carboniferous.^ The Donegal (3) and 

 Connaught (4) granites are of a totally different character. They 

 are essentially of a metamorphic type, being bedded and, in Donegal, 

 interstratified with limestone ; ^ they do not intrude into, but form 

 part of the great mass of gneiss, schist, quartz-rock, and limestone 

 among which they occur.^ 



If two geologists were to set to work to investigate the origin of 

 granite, and if one were to locate himself in Leinster and the other 

 in Donegal, the Leinster geologist could bring forward the most 

 convincing proofs of the intrusive character of granite, while the 

 Donegal observer could produce equally conclusive arguments in 

 favour of its metamorphic nature. 



1 am at a loss to understand how any one could explain the Leinster 

 granite by the metamorphic theory, yet the Donegal rock appears to 

 be but an instance of an advanced or perfected stage of that meta- 

 morphic action which is less fully developed in the varieties of 

 gneiss. Any geologist who has examined gneissose districts may 



^ Jukes, "Student's Manual of Geology," pp. 93 and 313. I think, ho-wever, 

 further proof is required as to its being of the same age as the rock Avhich alters the 

 Carboniferous Limestone near Carlingford : it rather differs in appearance and mineral 

 composition from the Leinster granite, containing other micas, and notably by the 

 occurrence in some places of albite (Haughton, Quart. Journ. Geol. Soc. vols. xii. and 

 xiv.), though I believe that this feldspar is not so important a constituent as has been 

 supposed. 



2 Brit. Assoc. Eeport, 1863 ; Scott, Journ. Geol. Soc. Dublin, vols. ix. and x. See 

 also Haughton, " On Granites of Donegal," Quart. Journ. Geol. Soc, vols, xviii. 

 and XX. 



3 There can be little doubt that some intrusive granites do occur in Donegal and 

 perhaps largely in Connaught : we require further information on this point ; a red 

 patch on a map, lettered G for granite, does not teach us much. 



[" Stratified eruptive rocks." See Forbes, " The Microscope in Geology," in this 

 number, p. 515. — Edit.] 



