886 MR JOHN S. FLETT ON 



of (fykes, which in some points are quite different from any others in the Orkneys. 

 The most interesting is that just below the house of Binniaro. It is porphyritic, and 

 the only Orkney camptonite to show phenocrysts of felspar. These are numerous, well 

 shaped, free from inclusions, and belong to a labradorite of average composition. They 

 show albite and pericline twinning, and are tabular on the brachypinakoid. The other 

 crystals of the first generation are olivine, altered into serpentine, and augite, of a pale 

 greenish variety, in idiomorphic crystals, aggregated into nodular groups and decom- 

 posing into chlorite. It has an extinction angle of 43°. The groundmass consists of 

 felspar (labradorite of a rather acid kind) and augite largely chloritised, and is filled with 

 skeleton growths of magnetite. The felspar is "05 to '01 mm. in section, and up to 

 •5 mm. in length. The crystals lie scattered irregularly through the slide ; their form 

 is sharp and perfect. Between them lies the augite in grains of irregular form, mostly 

 altered into chlorite, and evidently later than the felspar in crystallisation. The struc- 

 ture hence is intersertal, and the rock approaches closely to the diabases. The associa- 

 tion of diabase with camptonite is too well known to call for any remark. The usual 

 sequence of crystallisation in the groundmass is, in this instance, reversed (see photo- 

 graph No. 2, PL III.). 



The dykes associated with this one are intermediate in character between it and 

 the normal camptonites. The nearest one is a fairly typical camptonite, rather rich 

 in augite, but with brown hornblende in smaller quantity, and panidiomorphic in 

 structure. Their chilled edges have more resemblance to diabase, being fine grained, full of 

 magnetite skeletons, and consist of small lath-shaped idiomorphic felspar, with chloritised 

 augite between. 



Chemical Analyses of the Camptonites. 



The dykes selected for analysis were : — 



1. The dyke at Eennibuster. 



2. A dyke from the Wart of Skaill, Sandwick ; very typical of the West Mainland 



dykes, with phenocrysts of decomposed olivine and a groundmass of augite, 

 hornblende, and plagioclase. 



3. The west dyke at Hoxa. 



4. The dyke approaching diabase at Binniaro Firth. 



With them a series of typical selected analyses is given for comparison. 



5. Camptonite, Maena, Gran (Brogger, VI., p. 26). 



6. Camptonite, Egge (Brogger, VI., p. 26). 



7. Camptonite, Campton Falls (cited from XIX., p. 235). 



8. Camptonite, Montreal (cited from XIX., p. 235). 



It is to be noted that in 1 and 2 the alumina carries a considerable amount of 

 titanic acid, which was not estimated. Another feature of the analyses is the high 



