524 Correspondence. 



remember to have seen true gneiss wliicli, in a hand specimen, it 

 would be impossible to distinguish from granite. 



Now I ask — Are we to suppose that, notwithstanding the vast 

 difference between their modes of occurrence in the field, the 

 granites of Leinster, and the granitoid rocks of Donegal must have 

 had a like origin, merely because they have a somewhat similar 

 mineral composition, both containing quartz, feldspar, and mica ? 



But have these rocks an identical mineral composition ? So far as 

 my experience goes, most assuredly not. They vary in appearance, 

 texture, and mode of aggregation of the component minerals ; the 

 quartz has a different look, difficult to describe, but once seen and 

 observed, not easily to be forgotten ; but above all, they differ widely 

 in their feldspathic constituents, for while the intrusive granites are 

 orthoclasic or, as in Down, sometimes albitic (and, let it be remem- 

 bered, albite is as highly silicated as orthoclase), and the uncrystal- 

 lized feldspathic paste is always highly silicated, the granitoid rocks 

 on the other hand contain, notwithstanding the presence of free 

 quartz, a large proportion of basic feldspars, of which oligoclase is 

 the most recognizable, and the feldspathic paste is basic also, ap- 

 proaching oligoclase or anorthosite ^ in composition. 



During a recent visit to Scotland I had these views fully con- 

 firmed by the facts which I observed there. The intrusive granites 

 of Arran are extremely like those of the Mourne district, while 

 many of the Highland rocks appear to pertain to the metamorphic 

 type. As my visit was very hurried, I cannot now commit myself 

 to details ; neither shall I say anything of the intrusive and meta- 

 morphic characters of the hornblendic series of rocks, such as green- 

 stones, syenites, and hornblendic schists. 



The views now put forward are only suggestive : my field of 

 observation has been too limited to warrant my entering into gene- 

 ralizations, but I trust they will tend to elicit further opinion on this 

 important subject. So long as our knowledge is added to, it matters 

 little whether these views are corroborated or refuted by such in- 

 vestigators as Forbes, Haughton, Hunt, and Sorby, men who combine 

 the highest chemico-mineralogical attainments with great knowledge 

 of physical geology, accomplishments which unfortunately do not 

 often co-exist in the same individual. 



In conclusion, I think the last passages of Mr. Forbes's paper 

 (Geol. Mag. Vol. IV. pp. 442 — 444), deserve the serious attention 

 of every one who may be inclined to go in for the metamorphic origin 

 of all granite. 



I am, Sir, your obedient servant, 



W. H. Staopoole Wbstbopp. 

 Blackeock, Dublin, October 5th, 1867. 



Since writing the above, I have looked into Haughton's Manual of 

 Geology, and find that I have been anticipated in suggesting a two- 

 fold origin for granite. In that work (p. 45) the terms hydro- 

 metamorphic and pyro-metamorphic are proposed. I fear that the 



1 Geol. Keport, Canada, 1854; and Bigsby, Geol. Mag. Vol. I. p. 157. 



