THE TKAP DYKES OF THE ORKNEYS. 889 



and without doubt many of the microliths enclosed in the felspars of the camjrtonites, 

 and often taken for apatite, are of similar nature. The glass is in places perfectly 

 structureless and isotropic, elsewhere is devi trifled and turbid, and in polarised 

 light shows a granular appearance. Very occasionally it encloses ill-defined 

 radiating skeletons of felspar. Apatite, magnetite, and ilmenite are the accessory 

 ingredients. 



The dyke already mentioned as rising along a fault fissure on the west side of the 

 Wart Holm of Copinshay is a monchiquite, remarkable for the large hornblende pheno- 

 crysts it includes. These may be as large as 1\ inches by \\ inches. Owing to their 

 perfect cleavage they are difficult to obtain in section, but they seem to be corroded, 

 often zonal, and almost free from enclosures. Decomposed olivine and idiomorphic 

 violet-brown augite, with sometimes a bright green centre, are the other phenocrysts. 

 The groundmass is fine grained, and resembles that of the Grainbank dyke, consisting of 

 titaniferous augite and rather plentiful biotite. Hornblende is confined to the rather 

 numerous ocelli, where it is mingled with felspar in fine radiating fibres. A partly 

 devitrified glassy base fills up the interspaces between the other minerals, and not 

 unfrequently shows in polarised light the presence of a small quantity of ill-developed 

 felspar. 



The Nature of the Groundmass of the Monchiquites. — This is a subject to which 

 attention has recently been directed by Professor Pirsson (XL, p. 680), who, in a very 

 able paper, has advanced the view that it is in reality primary analcite, which he 

 considers an essential mineral of rocks of this class. It had previously been regarded 

 as a glass, which, owing to its richness in alkalies and alumina, had a very marked 

 tendency to decompose into zeolites and analcite. Lindgren had previously (XII., p. 51) 

 called attention to the presence of this mineral in apparent phenocrysts in the analcite 

 basalts of the Highwood Mountains, Montana, and also in a second generation in the 

 groundmass. He suggests that the mineral could have formed from igneous magmas 

 in presence of water, and crystallising under sufficient pressure to retain it. Still 

 more recently Cross (XIIL, p. 684) has described certain "analcite basalts," in which 

 the analcite, though the last product of crystallisation, in many ways resembles the 

 phenocrysts of porphyritic rocks. He, too, considers it a primary mineral. 



The grounds on which Pirsson bases his opinion are, that it is in itself improbable 

 that rocks so basic in composition, solidifying under great pressure, should produce any 

 glassy material, while under similar circumstances the acid dykes of the same districts 

 are always crystalline. That it is not impossible, however, is proved by the frequent 

 tachylite selvages of basalt dykes, and the association of variolite with diabase. Com- 

 parison of the limburgites with the basalts will show tha.t it is not merely the percentage 

 of silica a rock contains which determines the abundance or frequency of glass in the 

 groundmass. The chemical analysis of the glass in a monchiquite from Brazil has, as 

 Pihsson shows, a striking resemblance to analcite, and he considers it has been made 

 with great care and skill on excellent material. We must remember that the ground- 



VOL. XXXIX. PART IV. (NO. 33). 6 X 



