882 MR JOHN S. FLETT ON 



The groundmass consists of brown hornblende, brownish-violet augite, plagioclase felspar, 

 and glassy base. The hornblende preponderates, and is commonly in parallel growth 

 with the augite, and both have sharp crystalline form. The felspar in irregular masses 

 fills up the interspaces. The glassy base is not abundant, is turbid with decomposition 

 products, and filled with colourless, ramifying microliths, which appear to be felspar. 

 Its presence indicates that the rock is a transition to the monchiquites, where such a 

 material is a constant constituent. Rounded pale ocelli, consisting of felspar prisms 

 mixed with a little hornblende and much glassy matter, are scattered through the 

 slides (PI. II., fig. 4). 



The other dykes contain the same minerals, though the phenocrysts are smaller, 

 and the groundmass richer in felspar. Towards the edges of the dykes there is a 

 tendency to the development of much glassy material, which, with the absence of felspar, 

 makes the sections very similar to those of some monchiquites. 



In all the South Ronald shay dykes, but particularly in the west dyke of Hoxa, 

 sections of a colourless isotropic mineral, clear, transparent, without regular cleavage 

 or trace of crystalline form, are to be observed. It rarely shows any double refraction, 

 and agrees in all its characters with analcite. The manner of its occurrence is 

 pretty varied. It is abundant in the pale ocelli, mixed with felspar and pale, turbid, 

 decomposed glass. There it is usually central, but appears in the ocellus itself, and not 

 in the miarolitic cavity, which is occupied by calcite. It is surrounded by the glassy 

 base, and merges into it gradually on every side. It is apparently formed by the 

 decomposition and hydration of areas of a glass, which, as Pirsson has shown (XL), 

 has a close resemblance in chemical composition to analcite. In scattered spots in the 

 groundmass it is also seen, and is here of similar origin. The tendency of the still 

 liquid magma to aggregate into globules after the crystallisation of the hornblende and 

 augite of the groundmass which has given rise to ocelli, has produced also smaller 

 rounded patches of glass, mixed with a little felspar at the edges, which readily change 

 to analcite. Within the larger phenocrysts, and especially the honeycombed, corroded, 

 nuclei of hornblende, analcite is abundant, occupying sac-shaped cavities. In many 

 cases, around it are a few felspathic needles, lying in a little turbid glass with 

 hornblende in parallel growth with the enclosing crystal, and it may be a little 

 magnetite. These are, in fact, glass cavities decomposed and reconstituted into 

 analcite. At the periphery, the hornblende and magnetite have crystallised out with 

 a little felspar, leaving in the centre an area of glass. In other cases, the cavity is 

 apparently entirely filled with analcite, or analcite mixed with calcite, which may 

 surround it or be enclosed in it, or each may occupy one side. Similarly, a rounded 

 patch of analcite may lie in the groundmass, and may exactly resemble a corroded 

 phenocryst. Out of a large number of sections the best example of this is photo- 

 graphed in fig. 3, PI. II. In view of the suggestion made by Lindgren (XII.), and 

 adopted by Cross (XIII.) and Pirsson (XL), that analcite may be a primary por- 

 phyritic ingredient in rocks allied to these, I have given special attention to these 



