898 



MR JOHN S. FLETT ON 



position is very similar to that already described in the former rock, fibrous zeolites, 

 with straight extinction parallel to the peg-structure, forming first in a net-work, and 

 spreading afterwards through the mineral. In the freshest parts of the slides it fills up 

 all the spaces between the other ingredients of the groundmass, and leaves no room 

 for any later mineral or for a glassy base. But where the rock is more decomposed, 

 calcite, with chlorite, brilliantly polarising zeolites, and an isotropic mineral (analcite ?), 

 in a mosaic of irregular grains and fibres, form the interstitial material. These have, no 

 doubt, resulted from the final disintegration of the melilite and the adjacent minerals. 



The rock contains a few vesicles filled with calcite, chlorite, and zeolites. In the 

 hand specimen it is fine grained, non-porphyritic, and in places much decomposed. It 

 closely resembles the monchiquites, but is distinctly paler in colour, being rather greyish 

 green, while they are dark green. 



A chemical analysis is given below (No. l), and beside it certain others for com- 

 parison. 



Chemical Analyses of Melilite Eocks, etc. 





1. 



2. 



3. 



4. 



5. 



Si0 2 



33-87 



42-51 



35-54 



33-89 



35-84 



Ti0 2 



2-12 





2-03 



0-64 



8-85 



Al 2 O s 



15-25 



12-04 



11-72 



9-93 



10-48 



Fe 2 3 



2-37 



2-67 



5-86 



1563 



7-25 



FeO 



5-15 



7-52 



5-99 



• •• 



6-62 



MnO 



•32 



•83 



•32 







MgO 



12-52 



12-00 



13-56 



16-14 



12-95 



CaO 



14-43 



11-83 



15-83 



1519 



10-90 



Na 2 



1-41 



2-75 



1-91 



2-86 



3-53 



K 2 



1-02 



2-15 



2-24 





1-51 



H 2 



2-47 



2-96 



1-67 



2-90 



... 



p 2 o 5 



•99 







1-41 



... 



C0 2 



8-64 



3-46 



4-30 



1-41 



2-84Cr 2 3 



100-36 



100-53 



100-97 



100-00 



100-77 



Sp.G. 



3033 



2-905 



3-052 



3015 



3-051 



1. Melilite monchiquite, Long Geo, Holm, Orkney. 



2. Monchiquite, Grainbank, Kirkwall, Orkney. 



3. Alnbite, Naversdale, Orphir, Orkney. 



4. Melilite basalt, Hochbohl bei Owen. 



5. Nepheline melilite basalt, Riedoschingen, Hegau, Baden. 



(Nos. 4 and 5 cited from Rosenbusch (XIX., p. 360).) 



A comparison with 4 and 5 will show that the melilite monchiquite from Orkney 

 has a sufficiently close similarity to other well-known rocks of the melilite group. It is 

 in a state of more advanced decomposition, as witness the high percentage of C0 2 , which 

 is partly combined with magnesia, as it was noticed that only a small amount of C0 2 was 



