890 MR JOHN S. FLETT ON 



mass is decomposing into zeolites, and (XVI. , p. 452) that the solution from which the 

 material was precipitated had been diluted to a specific gravity of 2 - 3, which is less 

 than that of most natural glasses, so that the material must have in any case contained 

 a disproportionate amount of these secondary minerals. A single analysis is, moreover, 

 a slender basis on which to build so important a theory. Two analyses of the material 

 from Lindgren's rocks, which also are admittedly rich in analcite, are adduced to 

 support the hypothesis, but they seem to differ too much from one another to be 

 suitable for the deduction of a mineral formula. Finally, in Pirsson's view, the 

 ready gelatinisation with acids, and the weak and irregular double refraction, point to 

 this substance being really analcite ; but these points have long been known and not 

 considered incompatible with the properties of a natural glass such that it readily 

 decomposes with formation of zeolites (Hunter and Rosenbusch, XVI., p. 452). 



As a result of my examination of the Orkney trap dykes, I have been led to form 

 the opinion that this material is really a glass, which, owing to its composition, very 

 readily decomposes, with formation of zeolites and particularly of analcime. Where 

 analcime can be identified, as in the west dyke of Hoxa, it is quite different in appear- 

 ance from the glassy base, being clear and transparent, while the latter is turbid and 

 full of microlites. In some rocks, as in the fourchite, this glass is perfectly clear, 

 fresh, and isotropic, though filled with felspar microlites, which are grouped in radiate 

 bundles very similar to those of the minettes. It may be colourless or pale brown. In 

 decomposition it becomes turbid, granular, semi-opaque. In polarised light it is then 

 filled with specks and fibres of undeterminable nature, which give it the appearance of 

 a devitrified material rather than of analcite. Finally, it becomes a granular mosaic of 

 calcite and analcite, mixed with various fibrous zeolites. 



Moreover, its occurrence is that which is distinctive of a glassy base. In certain of 

 the camptonites, as, for example, those of Hoxa, an isotropic matter is abundant at the 

 •edges of the dykes. Towards the centre felspar increases in abundance, and the base 

 diminishes, but does not disappear. The sections of the edges have all the characters 

 of hornblende monchiquites. Yet if the base is here analcite, it cannot be analcite 

 which in the centre has been left after the crystallisation of the felspar. On the other 

 hand, the passage from monchiquites to alnoites will be shown to be a very gradual one. 

 There are forms of the latter rocks which at the centre show melilite, at the edges only 

 a glassy base ; other dykes contain a granular glass throughout. The transition is so 

 perfectly gradual, that no line can be drawn to separate the " analcite rocks " on the 

 one side from the felspathic and melilite-bearing rocks on the other. 



Should ever this hypothesis be established, it must rest on a broad basis of chemical 

 facts. The attempt which I made to separate out the groundmass of the Orkney dykes 

 failed to yield material of suitable purity. It is difficult to see how it could be obtained, 

 as the undoubtedly secondary analcite of the amygdules should certainly be first removed. 

 The'rock which Kosenbusch and Hunter investigated (XVI.) is far more suitable for 

 this purpose than any other I have seen ; but it should not be forgotten that this is 



