THE TRAP DYKES OF THE ORKNEYS. 



887 



percentage of potash in 1 and 2, where it preponderates over soda. This is partly due 

 to the composition of the hornblende (see analyses, p. 892), but indicates also the 

 presence of a small amount of orthoclase. The high percentage of iron oxides, and the 

 high specific gravity, are due to abundant magnetite, pyrite, and ilmenite, as well as to 

 the hornblende and augite. No. 3 shows a transition to the monchiquites in the lower 

 silica and alumina, the higher magnesia, and the relative amounts of the alkalies ; but 

 in the titanic acid, and the large amount of iron oxides, it belongs to the camptonites. 

 This is owing to the abundance of hornblende, magnetite, and ilmenite. In No. 4 the 

 alumina is rather low, the iron oxides rather high, a feature of the camptonites, but 

 otherwise it approaches very closely to many olivine diabases. The alkalies are in less 

 quantity than in the camptonites, particularly the potash, and in this respect, and also 

 in their relative proportion, this rock is of a more normal type. To this is to be 

 ascribed the absence of the peculiar brown hornblende. 



ANALYSES OF CAMPTONITES. 





1. 



2_ 



3. 



4. 



5. 



6. 



7. 



8. 



Si0 2 



41-99 



4213 



3913 



45-96 



40-60 



42-05 



41-94 



40-95 



Ti0 2 







4-02 





4-20 



5-60 



4-15 



3-39 



A1 2 3 



17-58 



16-31 



11-38 



12-68 



12-55 



12-30 



15-36 



16-45 



FeO 



8-33 



7-93 



8-13 



7-94 



9-52 



9-52 



9-89 





Fe 2 3 



6-17 



6-43 



7-33 



7-63 



5-47 



3-81 



3-27 



13-47 



MnO 



0-29 



•50 



•42 



•61 







0-25 



0-33 



MgO 



8-03 



7-37 



8-64 



8-25 



8-96 



4-83 



5-01 



6-10 



CaO 



8-53 



9-62 



11-77 



8-36 



10-80 



11-55 



9-47 



10-53 



K 



2-81 



2-48 



1-93 



•98 



1-19 



1-11 



0-19 



1-28 



Na 2 



2-12 



2-27 



2-47 



1-88 



2-54 



2-18 



5-15 



4-00 



co 2 



1-80 



2-12 



2-41 



2-43 



2-68 



2-68 



2-47 





p 2 o 5 



. • ■ 















0-29 



H 2 



2-99 



3-16 



2-87 



4-10 



2-28 



2-88 



3-29 



3-84 



100-64 



100-32 



100-50 



100-78 



100-79 



98-51 



100-44 



100-63 



Sp. Gr. 



301 



2-98 



3-07 



2-96 









2-927 



The Monchiquite Dykes. 



Compared with the camptonites, the monchiquites are few in number, but they are 

 of general distribution, and occur in all the principal areas, with the exception of the 

 west coast of the Mainland. 



A typical monchiquite forms a small dyke in the shore of the Peerie Sea, Kirkwall, 

 below the mouth of a spring which issues from the fields of Grainbank. It is a fine- 

 grained dark green rock, with few spots of calcite. Under the microscope (PL III. fig. 3) 

 it shows olivine in sharply idiomorphic crystals, altered into serpentine ; these contain 



