THE TRAP DYKES OF THE ORKNEYS. 



891 



an extreme member of the series, and, in its richness in alkalies, stands in striking 

 contrast to the rocks which have later been associated with it (Analysis 5, below). 



CHEMICAL ANALYSES OF MONCHIQUITE DYKES. 





1. 



2. 



3. 



4. 



5. 



SiO, 



42-51 



42-46 



43-74 



43-50 



46-48 



Ti0 2 





2-47 



2-80 



2-10 



0-99 



A1A 



12-85 



12-04 



14-82 



18-06 



16-16 



Fe 2 3 



2-67 



2-19 



2-40 



7-52 



617 



FeO 



7-52 



5-34 



7-52 



7-64 



6-09 



MnO 



0-83 



0-16 









MgO 



12-00 



12-40 



6-98 



3-47 



4-02 



CaO 



11-83 



12-14 



10-81 



13-39 



7-35 



Na 2 



2-75 



1-21 



3-08 



2-00 



5-85 



K 2 



2-15 



2-68 



2-90 



1-30 



3-08 



co 2 



3-46 



•55 



1-50 



1-22 



0-45 



PA 





•84 



0-64 







H 2 



2-96 



4-03 



2-94 





4-27 



100-53 



99-51 



100-23 



100-20 



100-91 



Spec. Grav. 



2-905 



2-94 



2-914 



3-051 



2-736 



1. Grainbank, Kirkwall. 



2. Willow Creek, Castle Mountain (Pirsson, XIV., p. 115). 



3. Monchiquite, Rio de Ouro. Rio de Janeiro (XVI., p. 464). 



4. Hornblende Moncbiquite, Magnet Cove, Arkansas. Analyst, W. A. Noyes (XXL, p. 295). 



5. Moncbiquite, Santa Cruz Babn, Rio de Janeiro (XVL, p. 464). 



The monchiquite dyke of the Wart Holm, Copinshay, carrying the large phenocrysts 

 of hornblende, presented a specially favourable opportunity to obtain this mineral pure 

 for analysis. About 20 grammes were broken out of various crystals, hand picked, 

 crushed, and thrown into borotungstate of cadmium. The principal precipitation took 

 place on dilution to a specific gravity of 3'15, and, on microscopic examination, proved 

 to be pure hornblende, perfectly fresh, and free from enclosures. The material had 

 previously been purified by passing over a strong electro-magnet to remove the magnetite. 

 The analysis is given under No. 1. 



As this rock carries a little felspar in the groundmass and in the ocelli, it approaches 

 closely to camptonite, and the composition is probably the same as that of the horn- 

 blende in the mass of the dykes. From these pure material could not be obtained, 

 owing to the frequent intergrowths with augite and admixture of its decomposition 

 products. The average given in the third column shows that this is a typical basaltic 

 hornblende. 



The Alnoites and Biotite-Monchiquites. 



To this group belongs a small number of dykes, so far as at present known confined 

 to the district between Kirkwall and Stromness. They have all a very northerly 



