258 H. J. Seymour — Blue Amphibole in Hornblende. 



The rock in which it occurs appears as a dyke (in Upper 

 Silurian sediments) ^ on the shore one-eighth of a mile south of 

 the point where the letter B in South Bay is engraved on the map ; 

 three miles south-east of Portaferry (1" sheet 49), and near the 

 spot where the small road south of 'Tara Fort ' reaches the coastline. 

 On the original 6 inch working maps of the Survey it is represented - 

 as a double dyke, with the ends of each portion in parallel contact, 

 the northern limb forming a semicircular bend in the middle and 

 enclosing a small area of sedimentary rocks. The combined width 

 of the two portions is approximately 20 inches. 



In the hand-specimen tlie rock is of a dull greyish-green colour, 

 and possesses a medium-grained crystalline structure. Bronzy mica 

 is the mineral most readily noticeable, and a few octahedra of 

 magnetite may be seen with the aid of a pocket lens. Its specific 

 gravity was found to be 2*85 in its present slightly decomposed 

 condition. Under the microscope it is seen to consist essentially of 

 a dark mica, with subordinate green hornblende, and an aggregate 

 of felspar crystals. Accessory and secondary minerals also present 

 are apatite, magnetite, a blue amphibole, chlorite, and several 

 calcareous pseudomorphs, apparently after augite or hornblende 

 phenocrysts, more probably the latter. 



Fig. 

 Fig. 



Minerals. — Hb. green hornblende, m. biotite. Ap. apatite. £. amp. blue 



amphibole. 



1 shows two large hornblende phenocrysts, with additions of secondary blue 

 amphibole on some of their terminal faces, x 45 diams. 



2 shows four crystals which exhibit the typical mode of occurrence of the 

 blue amphibole. That in the S.E. quadrant shows a basal section with the 

 elinopinacoid of the primary' green hornblende replaced by the blue amphibole. 

 The crystal in the S.W. quaclrant shows a narrow pale-blue zone between the 

 green hornblende and the normal blue amphibole. x 45 diams. 



^ These sediments were originally regarded as Lower Silurian. The recent revision 

 (by Mr. P. W. Egan), however, "shows them to be Upper Silurian. Eeport of 

 Director -General of Geol. Survey, Appendix to 44th Eeport, Dept. of S. and A., 

 p. 365. 



- By j\rr. "W. A. Traill, who mapped the dykes in this district. See Memoir of 

 Geol. Survey to accompany 1" sheets 49, 50, and 61 (1871), pp. 6 and 43. 



