896 MR JOHN S. FLETT ON 



olivine, augite, biotite ; 2nd augite, melilite. In all probability there was no glassy 

 residuum. 



A rock, which appears to be a much decomposed alnoite, forms a small dyke which 

 crosses the burn of Ireland, Stenness, where the two tributary burns unite. This dyke 

 forks into two branches. It has all the features of the alnoites, but in the groundmass 

 melilite is not recognisable, although in all probability originally present. 



It has been already remarked that at the edges of the dykes the alnoites tend to 

 develop a glassy groundmass, biotite being present in small quantity, and melilite 

 absent. Sections taken from the thin veins which connect the dykes at Eennibuster 

 burn have a totally different appearance from those of the dykes themselves. Large 

 phenocrysts of olivine (altered into serpentine) and of augite lie in a dark brown turbid 

 groundmass, evidently at one time glassy, but now decomposed and devitrified. No 

 biotite and no melilite appears. In the groundmass are microliths of augite and 

 minute brown grains, which may be biotite. The glass has rounded paler spots, which 

 recall the ocelli of the camptonites, and the groundmass is distinctly fluidal. 



On Rennibuster Point, a short distance to the west of the alnoite dyke first 

 described, occurs another, a little over a foot broad, and with the same northerly 

 trend. In section it is the freshest of the monchiquites and alnoites of the Orkneys, 

 and the numerous phenocrysts of olivine show only slight decomposition into serpentine, 

 and are sometimes absolutely unaltered. There are many phenocrysts of augite which 

 exactly resemble those of the alnoites. A deep brown biotite, in small, fairly idio- 

 morphic crystals, is rather plentiful. It encloses only apatite, magnetite, perofskite, 

 and may surround porphyritic olivine or augite. Only very rarely do the augites of 

 the groundmass encroach on it. It has a narrow, deep brown border. In the ground- 

 mass we have many small augites embedded in a brown granular glass. A careful 

 search has revealed a few sections which resemble melilite. It is perfectly idiomorphic 

 and sharply rectangular. Apatite and perofskite are frequent accessories. There is 

 one large brown corroded crystal of melanite, with traces of twelve-sided outlines and 

 a broad black border. This is a glassy alnoite, which very closely approaches in 

 microscopic character certain of the monchiquites. 



In the table on next page an analysis is given of the alnoite of Naversdale, the only 

 one I have so far been able to overtake. It is accompanied by the principal analyses 

 of similar rocks elsewhere. 



Melilite- Monchiquite. — This name I propose for a rock which occurs in a small 

 dyke at Long Geo, Holm, and has been already mentioned as crossing a small 

 fault (p. 872). In consequence of the irregularity in its course, its true direction 

 cannot with certainty be established. It is in every respect a monchiquite, except 

 that the groundmass here consists wholly or in part of melilite in plates of irregular 

 shape. 



It contains phenocrysts of olivine, decomposed into calcite, pale serpentine, and 

 magnetite, and enclosing brown octahedra of perofskite. The olivines are rounded, and 



