THE TRAP DYKES OF THE ORKNEYS. 



897 



on their surface are implanted grainsLof augite^ and biotite in' small' scales. j Augite occurs 

 in few small phenocrysts, purplish-brown, idiomarphic,. not. markedly, zonal, i and 

 resembling those of the monchiquites rather than <ofi the alnoitea* The groundmass 

 consists of augite, biotite, and melilite..-^ The augites are in small prisms 5.to<10 (times 

 as long as broad, brownish-purple/idiomorphie,. sometimes, twinned- in,- 1. -100, frequently 

 showing hour-glass structure, and, with a, :high obliquity of extinction ;(44°)<)i They, ace 





r <-H — — ^ 



-r - — H 



- — . .■ .'.ii u VI J:', 



I ■• ) . 1 ■ . 





, 1. 



2. 



3. 





■: 



i ft ,: 



1 ' 1' ■' (|- ':< 





[lid molct 



> i : . 1 J ii.' ■ 



■ .i <. . : Jv. 



Si0 2 



35-54 



35-91 



24-19 



Ti0 2 



' 2:03 



0-23 



-tr. -.;[, 



A1 2 3 



11-72 



11-51 



12-00 



Fe 2 3 



5-86 



2-35 



6-45 



FeO 



5-99 



,5 -.38. 



9-32 . 



MnO' 



0-32 





1 1' 



MgO' 



13-56 



17-54 



14-07 



CaO 



15-83 



13-57 



17-37 



Na 2 



1-91 



1-75 



1-99 



K 2 



2-24 



2-87 



3-06 



H 2 



1-67 



9-40 



5-16 



p 2 o 3 







3-96 



C0 2 



4-30 





, 2-77 



l60-97 



100-51 



101-16 



Spec. Grav. 



3-052 



3-02 



; 315 







rj 



1. Alnoite, Naversdale, Orkney. I i I 



2. Alnoite, Ste. Anne de Bellevue, Montreal (anal. Lb Rossignol,| XVIII., p. 271|). I 



3. Alnoite, Alno (XIX., p. 235). j I I 



The accordance between the first and seqond is exceedingly close. The titanic acid was estimated by the 

 method described by Hillebeand (XX., p. 45), and must be contained in the perofskite, titaniferous 

 magnetite, and augite of the groundmass. The Canadian rock carries magnesia partly as carbonate. 



; l i 



not dichroic, and almost perfectly undecomposed. They lie irregularly dispersed, or 

 groups in radiating rosettes. Biotite occurs only in small scales of irregular form, dark 

 brown colour, and intense dichroism. It is not, however, like that of the alnoites, being 

 less clear yellow, with a not colourless, but distinctly yellow-brown. It resembles the 

 later biotite in these rocks, and is almost perfectly uniaxial. Around the olivines and mag- 

 netites, it often forms a narrow border, and in the groundmass; it is mingled with augite 

 and melilite. Except its less idiomorphism, there is nothing to distinguish it from the 

 biotite in such a monchiquite as that of Grainbank.! The. melilite was the last mineral 

 to crystallise, and is destitute of crystalline form; its 1 irregular masses enclosing all the 

 other ingredients. It is paler a'm ; d' fresher than in the alnoite of ' 'Naversdale- almost 

 colourless, with very perfect pieg-str'ncture (see 1 photo,' PL III. fig. 6), ' though the 

 cleavage is often indistinct, no 1 dichroism, and gives polarisation; colours of dark grey; 

 about the same as apatite, or slightly lower. ' Its -optical -sign "is' negative. Its decdm- 



V0L. XXXIX. PART IV. (NO. 33). 6 Y 



