92 MR. GAEDINER AND PEOF. EEYNOLDS ON THE [Feb. I912, 



Allusion may here be made to an exceptional rock-type seen only 

 at (30), a spot about a quarter of a mile south of Kuocknamuck, 

 where it appears to be intruded between the lime-bostonite and 

 the Ked Sandstone. In a hand-specimen it is a pinkish rock, with 

 pale-green felspar-phenocrysts and small ill-delined dark patches. 

 Sections show the rock to be principally composed of almost 

 square felspars with simple twinning. Scattered through the 

 ground-mass are larger weathered felspars. Albite is present ; 

 but the majority of the felspars, large and small, are orthoclase. 

 The dark patches prove to be chloritized pseudomorphs after horn- 

 blende. Epidote and calcite are also present as alteration-products. 

 The rock may be called a hornblende-felsite. 



(b) The Lime-Bostonite. 



This retains a very uniform character throughout the whole length 

 of its outcrop. It is a purple or pinkish rock, very compact and fine- 

 grained, and hardly ever showing any phenocrysts. The scarcity 

 or abundance of its amygdules is its sole varying characteristic 

 in the field. As a rule, only a few small scattered amygdules 

 are seen ; but sometimes, especially near the base of the sill, as 

 at (134), they are very plentiful. Quartz is by far the commonest 

 mineral forming the amygdules. In thin sections the lime-bostonite 

 is very uniform in character, consisting of a felted mass of small 

 felspar-needles associated with much iron-oxide (sometimes mag- 

 netite, sometimes hsematite), much of which in either case is 

 probably secondary. This is, as a rule, very evenly distributed 

 through the sections in small patches; but it sometimes occurs 

 interstitially, and may, like certain ill-defined patches of calcite 

 which are sometimes (49) seen, represent ferromagnesian minerals, 

 no sign of such minerals in an unaltered state having been observed . 

 in these rocks. 



The felspar was determined as albite by Dr. Plett, who pointed 

 out that these Kilbride bostonites are more felspathic than the 

 spilites, containing from 60 to 70 per cent, of felspar. Felspar 

 phenocrysts in a much-weathered state are only rarely met with, 

 and no sign is seen of a glassy base. There is little trace of flow- 

 structure. These rocks show a considerable resemblance to the 

 lime-bostonite described by Dr. J. Y. Elsdeu ^ from the district 

 north-east of St. David's Head and still more to the keratophyre 

 described by Mr. Herbert H. Thomas - from Skomer. 



We have said that our lime-bostonite resembles microscopically 

 the St. David's rock described by Dr. J. V. Elsden (oj>. cit. p. 595). 

 The similarity in chemical composition is shown by the analyses of 

 the two rocks : — 



1 Q. J. a. S. vol. Ixi (1905) pp. 594- 



2 Ibid. Tol. Ixvii (1911) pp. 193-95. 



