Hyland — On some Epi-diorites of N. W. Ireland. 409 



The 10.40 7 °f orthoclase is presumably not present as an inde- 

 pendent felspar; hence, the analysis represents a felspar of the 

 constitution Ani Ab 2 .4, *. e. a felspar intermediate between andesine 

 and oligoclase. It would manifestly be more accurate to analyse 

 the felspar which occurs in the ground-mass of the rock itself ; but 

 no attempt has apparently been made in this direction, owing to 

 the difficulties the isolation presents. In order to do this, the rock 

 from Convoy was taken, and, after being powdered, treated with 

 weak acid in order to remove the small quantity of calcite present. 

 The secondary felspar was then separated by means of the 

 Sonstadt- solution : its specific gravity was found to be 2.645, 

 The analysis gave the following result : — 



100-20 100-00 2-11 71-49 22-31 



33 1 



The constitution of the secondary felspar is, therefore, An t 

 Ab 3 . 3 — in fact, the rearrangement of the original labradorite Ab! 

 An 3 has resulted in the formation of an oligoclase. We have, 

 accordingly, the association of uralite and oligoclase in these 

 rocks, and it is, hence, interesting to recall Gr. Rose's observations 

 upon the subject {vide supra). Owing to the metamorphism of the 

 felspar, a large amount of lime was set free, which induced the 

 formation of epidote and zoisite. The aggregates of these two 

 minerals are sometimes found piercing the bi-silicates in contin- 

 uous lines, and thus preserve the ophitic structure so characteristic 

 of dolerites. 



1 The Fe 2 O3 was estimated by titration : the amount is certainly high ; but the 

 sp. g. of the powder (2.645) shows that it cannot be ascribed to the presence of epidote. 

 2.59°/ of Feo03 would be equivalent to an admixture of about 5.8°/ D of epidote, which 

 would demand a sp. g. of about 2.9. (V. Cathrein " Ueber Saussurit," Zeits. f. Kryst- 

 vii. 241, 242. 



212 



